Most of those 700r4's came with the 208. Its a lighter transfer case with a chain drive. Most people are not fans of them but they are easy to get and have a 2.7:1 or so low range. The bad is that they are not as strong as a 205 but that might no matter in your lighter application. Another good one is the NP241. It was the replacement for the 208 and is stronger and you can get a yoke conversion kit for the rear.
It also depends on your wheeling. If your doing mud, get a 205. They are bullet proof and have a 1.96:1 low range. You will have to get an adapter for it to work on a 700r4 but it can be done.
I hate to plug another site over my own but www.coloradok5.com is a kick butt place for Chevy and Blazer info. Do a search over there and I bet you'll find your answers. :thumb: