Rear end suggestions

Ideally, if you have access to a lathe, you chuck it between centers, and using a dial indicator, you check the tube for runout.
When I didn't have access to a lathe, I made a pair of v-blocks with some angle iron and sealed ball bearings. I set the v-blocks on opposite sides of my 64 dart decklid, placed the driveshaft in the v-blocks, and using my mag base and indicator, checked it for runout. Ideally, you want only a few thousandths of an inch, but you could probably get by with 0.010". Incidentally, my professionally fabbed driveshaft for my pickup had at least 0.020" runout. I used a welding torch to straighten it better, after the local shop said that's as good as they get.
One other thing to check is the trans yoke engagement. If you don't have sufficient engagement/support the driveshaft will vibrate. My late friend, Brian, destroyed his transmission TWICE, by not having enough engagement and powering thru the harmonics until it exploded.