Got a brand new driveshaft in today, replaced the tailshaft bushing and seal, and installed it. The vibration is still there. I normally don't buy replacement parts without knowing that the part I'm replacing is definitely the problem, but I've been wanting to replace my old driveshaft for a while anyway.
I did notice the vibration starts at about 45 mph, and if I get it up to 55 and put it in neutral it changes frequency and seems to get slightly worse.
I will get it on jack stands next and check the tires for bumps/broken belts, check for play in left to right and up and down, and even rotate the rears to the front and see what happens.
Just gotta keep narrowing it down.
Russell