Eliminate one thing at a time starting at the easiest.
Find a known set of chevy rims and tire and put them on to eliminate or secure those two possibilities.
You also mentioned that your lever shocks worked with different resistance. (Check the oil levels.) That could also be a problem, but with no weight in the back and probably not noticable at all speeds, it seems a unlikely to me.
The drive shaft seems a likely suspect next. Balanced and good bearings?
If you haven't pulled the rear end and axles (did I read that you hadn't looked at the rear axle bearings yet?), I'd clear my concience of the rear end/bearings being the possibility. What about the axle side play?
