Does anyone know where I can get a hold of a 85 z28 rear end? OR know of a shop that could do it for a good deal....

 

What's so special about an 85 Z28 diff? The 10 bolts from 82 to 92 are all roughly the same. In 1990 they were upgraded to 28 spline axles from 26 spline. Around 86 the ring gear size was upgraded from 7-1/2" to 7-5/8". Not a big increase and they both fit on the same carrier.

If all you're trying to do is get a better gear ratio and possibly convert from an open diff to a posi, parts from any other year 7-1/2" 10 bolt will work and not just from third gens.