If you have a good workspace and jackstands, do your diagnostic tests on jackstands. Just raise the rear end (support the car using the axle housing, not the frame) and run the car with the tires in the air. The vibration will appear here just like it would on the road. Remove the tires, see if the vibration persists. Check/change the pinion angle, see if the vibration persists. Swap the driveshaft, etc.
Note: If your pinion angle is different from your bro's, I think it will most likely be a "flatter" approach than your bro's. My Cuda has the original small block leaf springs and it sits a little lower in the rear than stock. I needed some shims to correct the vibration that came from the sagging. I don't remember what angled shims I ended up with, but it was somewhere around 2º. Also, the tires and driveshaft swap are good things to test, but be aware that your bro's Cuda may have differently sized U-joints than you, and a driveshaft swap might not be easy.
Good luck, and let us know what you find.
