Paul,
I have a 91 XLT 5-speed with a 5.5" Superlift. After the lift installation there was vibration and a droning noise evident. My 4x4 shop diagnosed it as the driveshaft so a new custom one was built. It is about 4" longer than the stock one. The splined slip joint is teflon coated for shock absorbtion, the stock one does not have this. Also, a shaft seal is used instead of the the rubber accordion cover, and it has a zert fitting to add grease when needed. You might as well throw some new U-joints in there while you've got the driveshaft off.
My custom driveshaft cost me $250. It was well worth it. Gone are any vibrations or noises. It's good knowing you have a strong driveshaft when you're out on the trail. My old driveshaft only had about 1.5" of stub into the shaft, making for a weak connection. My new driveshaft has about 6" of stub into the shaft, which makes for a solid connection and gives plenty of travel for suspension articulation.
Hopefully you have an experienced driveshaft person in your area. It really needs to be custom fitted to your rig for best results.
I also had my pinion angle turned up to match the output of the transfer case. The Superlift spring perches that just bolted on set the angle too low, causing some vibration. With the proper angle set, the Superlift perches were welded to the axle, making them much stronger and safer.
DOGMAN
