I know you said they checked the uni joints, but this is all that I can think of. Do they have grease nipples? If so give em a squirt with a grease gun, if you don't have one ask your friendly mechanic to quickly give them a grease. If no change, try removing the rear tail shaft and go for a spin in front wheel drive. Need to eliminate the problem because if it's starting to happen at a slower speed, it's obviously getting worse.

I know with the transfer case that they put ATF in them if you have auto hubs. Reason for this is with regular, thicker oil, when cold, the front drive shaft will turn and if you had auto hubs, would engage them. If you can remove the filler bung and stick your finger in, you should be able to tell what fluid you have in there.