Replacing front and rear u joints fixed mine completely. I am running a 4 inch lift with man a fre radius drop brackets uo front. No pinion correction in the rear. Drop your shafts. Take them to the drive shaft shop and get them replaced if you haven't done so in 20 years. Especially before tackling the big possibilities.I've been chasing this exact same problem so please allow me to share my experience.....
I've rebuilt both front and rear pumpkins to include all seals, bearings, gears and lockers. I've completely rebuilt the TC to include all bearings, seals and re-geared both high and low sides. I replaced the front drive shaft with a DC shaft. I still have a vibration that starts at about 65 mph and becomes unbearable at 75 mph. I have the growl and vibration upon lifting off the gas pedal. The only thing I've accomplished is to move the vibration up the speed scale about 10 mph and quieted the growl a little upon lifting.
After reading this thread from start to finish I can say I've already done everything being suggested. I'd hate to see someone else go down this expensive path of tearing into the diffs and TC. Mine were done as part of a total rebuild of the driveline and suspension before we discovered the vibration issue. I do believe the problem is in the drive shafts. I've pulled the front and driven a few miles and a lot of the noise and vibration went away, but not all. I haven't tried pulling the rear after installing the front DC shaft. That will happen this weekend. I also plan to go to a little shorter lift to improve drive shaft angles. A rebuild of he rear shaft is in my future. Someone mentioned above that new u-joints in their rear shaft helped, if not cured their problem.
If nothing else, a few new data points.