trying to find source of vibration

I'll take a stab. If the back shaft was deemed to be "junk" then I would have expected a vibration long before it was total junk. Maybe the real problem is the rear end is failing. It could be many other things. Maybe put it on stands and take the rear tires off and let it spin, see if it shakes. You'll have to just keep looking. It may be as stupid as a broken brake drum.