Slight vibration

[dom14]

After driving the vehicle today, I realized that the vibrations were mostly felt at high revs. They seemed to have been getting worse. anyway, checked underneath this evening. Found that the bolts connecting the driveshaft to the rear diff were loose enough to be turned by hand. Going to tighten them up in a while.

What?!! That's not good news. Those bolts need to be torqued properly or else the uni joint & diff pinion won't be happy.
Sounds like you've found your problem, but might have done the damage already anyway.

After tightening the bolts, park the vehicle without handbrake on and feel if there's any up/down/side movement in the uni joint(even better when the rear wheels are jacked up)
or diff pinion/yoke. There shouldn't be any movements at all. If there IS, get the uni joint or diff pinion bearing done accordingly.