I'll try to explain this as best I can.

We put a 68 el mino posi rear into my 70 chevelle. The elmino's rear mounts didn't match the chevelle so he welded them in the right place.
Test drove it and it had a severe vibration at about 30. We thought it was the u-joint so, we went and got a universal. Test drove it again and this time, The driveshaft fell off the car. Everybody that I've talked to has confused the hell out of me. I don't no what to do or where to start. Any suggestions?
Thanks
missy

Sounds like pinion angle. Take to a qualified chassis shop and have pinion angle checked. May require 'redoing' the welding that was done.

There is no reason the mounts should have been relocated, all 68-72 housings are the same. If someone relocated the UCA or LCA mounts either the housing wasn't from a 68-72 or they screwed it up. Fixing it is not going the easiest thing to do, measure twice cut/weld once. You are going to need to measure pinion angle with tthe suspension loaded at ride height and then figure out where to reweld the mounts for proper angle.