highway vibration

I would just get a new axle shaft for the 8 1/4 before I did anything drastic. You'll want to repair that rear end anyway if you plan on selling it. The E-body rear is going to be way too wide for an A-body without narrowing it. I think I would sell it and use the money towards a correct width rear for the car if you can't live with the 8 1/4.