1999 tahoe 4wd ..... high speed vibration

Let me first say my Hoe is the poster child for all Tahoe troubles .... If it's listed on any forum then I have most likely have had that problem. My trouble now is a vibration or chatter most noticeable above 55 mph. I have rotated the tires and 3 days ago installed new u-joints. Seemed to help a little ..... until yesterday when I was about 2 hours from home the vibration came back strong and a minute later spit the drive shaft off the pig. Making a long story short( 1/2 day later), replaced the rear u-joint again, but the vibration is still there. It vibrates worst when I'm going over 60 and let off the gas. Even when put in N it's still there. Going down hill and letting off the gas is worse. If I play with the pedal and keep the drive system under pressure instead of allowing it to coast it's not as bad.

I have found this problem listed on many forums, but have yet to find the right answer.

99 4wd Tahoe 170000 miles, rebuild tranny at 92000.