Place jackstands under the differential. Remove wheels/tires. Run motor up to identify or eliminate wheels/tires as the culprit. Vibe gone, balance wheels/tires. Still there? Remove rear brake drums/rotors and go again. Vibe gone, balance rotors/drums. Still there? Remove axles and go again. Vibe gone, straighten or replace axle(s). Vibe still there, remove driveshaft, use plastic cup to contains trans fluid at the splines of the output shaft and go again. Vibe gone, problem in diff or driveshaft. Take driveshaft to good shaft builder and have it spun up. If shaft in balance, problem is in diff. If shaft out of balance, have balanced, re-install and go again. Still there, problem in trans/converter/flexplate/engine balance. Prop up rear of motor, remove trans/converter and go again. Vibe gone, transmission/converter problem. Vibe still there, engine balance/flexplate problem. Replace flexplate with known proper balance item for engine balance system used (internal/external) and go again. Vibe gone, flexplate balance problem. Vibe still there? Internal engine balance problem.

Now, I've written this sort of "tongue in cheek", but it does show a process I might use of going through things to find a vibration.