Drive train vibration. Scratching my head.

Hey Guys. Could use some help here. Drivetrain is LA 360/904/8.25. Combo has been in the car for over 10 years. About a year and a half ago I developed a vibration at about 55 and by 70 it is pretty heavy. It is only under power meaning if I take my foot off of the accelerator it stops. It is also not constant. It sounds like waaawaaawaaawaaa. This all seemed to begin with a gear and sure grip change from 2.76 to 3.55. I tried turning the driveshaft orientation even though I marked it. No change. Thinking it must be from the rebuild (new gears, bearings and crush sleeve eliminator) we went back in and checked the contact pattern again and it looked great. The only thing I noticed is that the pinion preload was at 14 inch pounds with a spec of around 30 but when we tried to adjust, we could not increase it and we were applying over 200 ft/lbs. After putting in fresh fluid with additive the vibration felt like it came in higher around 65mph. I can also feel the vibration seat of pants at 30+ but not pronounced.
I then started using 2 hose clamps and moving around the back end of the driveshaft in 90 degree increments to see if maybe it was out of balance. Vibration maybe changed a little but hard to tell.
At this point I am not convinced it is from the rear end but it was the only thing I changed.
I am not a drivetrain expert. I did the rebuild with a mechanic buddy of mine who has done quite a few similar rebuilds on GM's. He has not felt anything like this before.
Any help is appreciated.