I have pretty much the same truck, not a crew cab and it's a GMC but you get the idea. I would like to put a 2-4 drop on it one day but for now I went with some 2" drop shackles in the back. It came with the stock 20's also, so I think it's filling out the wheel wells pretty well now, plus the drop shackels are a lot cheaper and much easier to put on.
Here is a picture of my truck for referance, keep in mind that in this picture it does look like it's sagging in the back a bit but that is just because of the angle and the crappy launch ramp i'm sitting on but I think you'll get the idea.
Also, if you do end up buying a lowering kit, I highly suggest McGaughy's, they make the best products for GM's they have even been awarded by GM several times for the design quality. Here's a link to their site.
http://www.mcgaughys.com