The Shakin' GMC
 

I have a 1992 GMC Vandura 2500 conversion van. It has just over 103,000 miles on it. I use it for gigs with my band. I bought it back in 2003 with only 40,000 miles on it and it has given me seven years of really solid service. In the last year I've replaced the rotors and front bearings, replaced the lower ball joints, given it a full tune-up and flushed the cooling system.

I don't drive the van more than a couple times a month. Over the past six months I've noticed an increasing vibration that happens between about 45mph and 65mph. The vibration stops when I let off the gas pedal OR increase the speed by about 5mph.

The tires were just rebalanced last month and that didn't do anything to change the vibration. My next suspicion is something with the driveline. I don't have any experience troubleshooting a driveline. What should I check?

You already did it. You get a vibration when you are on the gas. There you go. You re U joints are bad.