You say you like dynamic positions. Then the Slav does not seem appropriate to me. I'd suggest the Nimzo-Indian but yes that will require quite a bit of theory to be learnt. Same for the offensive Kings Indian. The Queens Indian however seems rather dull often resulting in an endgame with 2 rooks, Queen, and usually a knight and bishop.
What to play against 1. d4

Nimzo-Indian: A strategically complicated opening, where black often tries to make the knight stronger than the bishop, and white is trying for opening the position at all costs, so his bishops will prevail
The Grunfeld is quite complicated, where black must attack the white pawn centre. If white keeps the pressure under control, then he often makes a strong attack.
The Kings Indian is quite closed, where black tries everything on the kingside. However, if he doesnt mate, he will lose because of the white queenside pressure
The Modern Benoni is perfect for the comeback player (I play it myself, and hate it). Black has to play extremely good moves if he doesn't want to be lost out of the opening, but it doens't matter practically, since there is alot of counterplay in lost positions (sometimes).
The Queens Gambit Declined is interesting but positional. But if white tries on the queenside, black has somewhat free hands on the kingside. A horrible opening if you play against Karpov, though
ViktorHNielsen:
Whats a good book to get a basic idea of the concepts behind the Nimzo? Ive been looking at games from GMs of the 50s-70s and the Nimzo is quite appealing.

Even though the question is not for me I'd like to suggest Sokolov's Nimzo-Indian book, The Strategic Nimzo-Indian, it only mentions the 1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. e3 variations but it still involves a lot of ideas.

I know alot of ideas from The Nimzo-Indian move by move. Borrowed it from the library though, so I don't know about the other books. To play the Nimzo succesfully, you must be able to play QGD positions I think

I play King's Indian a lot, and it looks like no matter how much material I'm down, I always get some counterplay.

You have a lot of choices for dynamic play against d4. Nimzo indian is my favourite, with both sides. It's one of the most interesting openings in all of chess. The only issue is that white can avoid it.
If you don't mind gambits, I would seriously look into the Albin. It can lead very dangerous attacks by black, only at the cost of a pawn.

Plutonia is right...when you prefer dynamic positions, then you should choose the Albins-Counter-Gambit, its second advantage is, that many white players in your class are not really prepared for it...and when you are booked up, you should get more than enough compensation for a single pawn.

Defenses such as the King's Indian, Dutch, Benoni, Old Indian,I feel do give black play, with the benefit that white can't really avoid these openings. Probably the Grunfeld too, but black has to remember precise variations.
All other black replies to 1. d4 would require knowing variations from several openings.
With the Nimzo Indian and Queen's Indian defenses, you also need to know how to play against the Catalan, which is quite a serious opening, you can take a look at the chapters of one Catalan book here:
http://www.jeremysilman.com/shop/pc/Powerful-Catalan-The-p3720.htm
Guess I'm sticking to Slav then
Simple enough that I won't be losing out of the opening because of not knowing variations

I really only play ...d5. I'm using the tarrasch atm because it's pretty straight forward and I don't need to do anything special to deal with alternatives to d4/c4.
I gave up on the Nimzo and Benoni because they both require an alternate defense to 1.d4. I'm having enough of a challenge playing one thing against 1.d4. Playing the Benoni straight-up is still an issue because the f4 stuff is extremely theoretical and leads to irrational positions. And then there is d4/Nf3 to deal with.
I don't like going very far into theory so I didn't go with the KID or Grunfeld. So that just left 1...d5. I play d5. They attack it. I defend it. Life goes on.
But I'm pretty sure the sharpest way to play against 1.d4 is to attack or get rid of the d-pawn somehow.

I'm a benoni player, and honestly i dont know if its a good choice since int quite a complex opening , but une thing can be said about that onening.
Draw its always quite unlikelly!!! Its for all win or total loss. I find that all my matches witht he benoni are quite interesting with fore on the boards.
That said you forgot to mentio the sister opening of the benoni. the benko gambit. Beybe you should give the benko a try, i have the suspect that fits your plaiing style.

I'm sure this isn't your style, but I have been working with the Dutch lately.
At first it was really kicking my ass. But I've started to understand it and it has been working better for me.
I think the Dutch is not used in more advanced play, but for lower play, it seems to catch people off guard.

Everyone stopped playing the Dutch for some reason. I don't know why. I think Nakamura was the last top player to use it.

It seems I remember reading that it was the general concensus that there were much better choices for black than the Dutch. And that it was mainly one of those openings that is resurrected once in a while by some master and played until everyone remembers how to beat it again.

Originally, I just picked it because I liked the f5 move and the possibilities for attack on the king side.
Then I quit it because it just is so difficult to make work for someone at my skill level.
Then I went back to it because I didn't like giving up on it in the first place.
If you want to learn the nimzo and are below 1800 i would suggest: (http://www.chessbase-shop.com/en/products/the_nimzoindian_defence-the_easy_way)
It focuses more on the ideas then an concrete move orders but does have a solid black rep.
I'm sure this isn't your style, but I have been working with the Dutch lately.
At first it was really kicking my ass. But I've started to understand it and it has been working better for me.
I think the Dutch is not used in more advanced play, but for lower play, it seems to catch people off guard.
im very anti dutch because the backyard professor touts it like its the second coming of christ, so it must suck.
my opening choices against 1. e4 is sicilian or marshall gambit.
however, as my OTB rating goes up a lot of players are playing 1. d4 almost exclusively. What am i supposed to play against this? I like dynamic positions which 1 d4 doesnt seem to have much of.. unless i want to learn a lot of theory.
So im basically stuck between:
Modern Benoni
Kings Indian
Nimzo Indian
Queens Indian
Ive been playing the slav for quite some time but too many of my games are draws or im just waiting for my opponent to slip up and i get a win.
Im about 1600 OTB rating, the two clubs i go to have an average of 1600-1700 rating, but i usually get paired against 1800+ and end up in a queen pawn game.