what do i have to look for to find out if one rear end is compatible with another rear end? I want to replace the rear end in my 97 blazer because it has a bad vibration. How do i find out the gear ratio and how do i find out if it is a gu6 rear end?

 

First are you sure it is a rear end? Have you checked the U-Joints? Chevy's in general don't go through rear ends unless they are run out of oil. Vibration sounds like a U-Joint to me. Maybe a axle bearing. But they are way cheaper to fix than replace the rear end.
The most accurate way to find gear ratio is to count the teeth on the ring & pinion divide the teeth on the pinion by the teeth on the ring gear. I believe if no one has changed them the codes are in the glove box.