What will i need?

I got a '08 Denali w/ 26s and i just want to lower the back of it to make it level. Question is what parts will I need to do so and where could i look for them? Thanks.

You could get a 2/3 Lowering kit form Belltech, Mcgaughy's, Ground Force, DJM or Eibach that will lower the front 2" and the rear 3" which will make it sit much better on your 26's.
Or like you said you can get Belltech 3" or McGaughys 3" rear coils only, but by doing that the rear may be lower than the front to some extent because the smallest "rear only" drop is 3", which will make the rear lower than the front, so I advise you go with a 2/3 lowering kit (a 4/5 kit or 3/5 kit will rub with 26's) kit from one of the above companies.
Check out www.autoanything.com or www.stylintrucks.com for these kits, but they're are many more sites that heve them so do some price comparisons.

thanks for the info, I found some rear coils for a tahoe, will those work on the denali w/ autoride?

Well you didn't tell me what you brand you have but I know that the Pedders 2425 (Google it if you want to price shop a bit) has rear only coils that will satisfy your wants, but it lowers the rear almost 3.5" which will def. put your rear lower than the front.
I don't think what you found will work for you becaue Eibach, Ground Force and Pedders are the only kits I know that have a AutoRide working option.
http://shop.rocksandracingllc.com/p...E1379.qscstrfrnt03?categoryId=18&productId=14