I have a 1994 Ford F-150 4x4 it has a solid front axle. Lately I've noticed that from 30-45 MPH there's harsh vibrating. At 50 it calms down. When engaging into reverse, there is a clunk before it catches and it's usually rough. Could this be a sign of a bad u joint causing slack somewhere? If so which one is best to suspect? There's 2 on the rear driveshaft, but I don't think the front ones beyond the rear shaft would be an issue because none of them move unless 4x4 is engaged. Any input? Does this even sound like a U joint problem? One of the U joints on the rear shaft is factory. So it's 22 years old. Lol opinions please!

 

It could be... Did the shift to reverse clunk just start? If so, crawl under it and see if the u-joints have any play. If not, I think you may have a tire out of balance.

Some vehicles just clunk when shifting to reverse. My 1977 has done it since new.

 

It's done it ever since I've had it. Has new transmission fluid in it. Idk the vibration isn't constant it's only at a certain speed

 

I am betting on a tire balance issue

 

I would check the U-joints first thing. Park the truck on level ground so it can't roll and put the transmission in neutral to take the tension off the u-joints. Twist on the main part of the driveshaft while you watch and grab the part on the other end of the U-joint. Also rock the shaft up and down and sideways and feel for play. This method works real well once you get used to it.