I have a 94 F150 2wd xtra cab. 2 piece d.shaft. Started feeling a thump when I pull off from a stop. Checked the ujoints and found 1 with some motion. Replaced all 3 joints. While shaft was out, I felt for motion in rearend. Which I felt some. I pulled the cover and found slop between the spider gears and pin. I replaced spider and axle gears, pin and LS clutches. Still had the thump. I couldnt figure it out for a few days. Then I ran across a TSB for 4x4 driveshaft slip yokes. When the diff flexes, the slip yoke should slide in and out. If the yoke binds a little, the torque prevents it from slipping. Then will release and make a noise. Mines 2wd but thought I would try it. Dropped the driveshaft from diff and pulled the slip yoke apart, cleaned it and lubed with teflon grease. Put it back together and guess what... Fixed. Just wanted to post my fix as it may help someone else save some time and money.

 

Great find. Thanks for posting your issue and resolution!