Sounds like you covered the basics
I'd be looking at the center support bearing going bad on the driveshaft (it's under a heat shield above the exhaust and not the easiest to check). The u joints get old and aren't serviceable so it's a driveshaft replacement if that's what's happening. Rule of thumb if the center support goes out, don't waste your time replacing it. You'll be doing the whole shaft in a couple more weeks/months when that one fails too (and it will).
That's really about the only other spinning thing down there that can be doing it, so it's worth the effort to check.