OK, I'm at wits end with this problem I have a 1974 F-250 4x4 with a dump bed. It ran great for years. Now the rear drive shaft has almost broken on me twice. This only happens when it is under load. Truck drives fine. Right now it is sitting outside my shop with a mangled u joint. After the first break I put it on the lift and found the right brakes locked up. Fixed it, and it happened again. Second time I changed the angle of the differential. It almost happened again but I caught it before it snapped and only have a messed up u joint. I need some suggestions on what else to try or look for. I have a fully stocked shop with a lift. I really need this truck to do heavy work or else it is no good to me.

 

Could you provide some addl info?
Is the u-joint breaking or are you wringing the driveshaft?
If it's only the u-joint , are you using aftermarket imported u-joints?
Did you purchase u-joints from Ford?
Most of the time when a driveshaft lets go it's because the brakes were still on and the driver put the coals to it. The driveshaft is designed to be the weakest link to prevent damage to the trans or differential. Lack of lubrication will usually help wipe out a u-joint, but if new they should have sufficient lube. Did anyone apply the parking brake with frozen brake cables and the brakes won't release?

 

usually when a drive shaft twists is when it on a bind ! make sure the back wheels spin freely when they are in the air ! make sure the parking brake is not hanging up and check the grease in the rear end !

 

the pin that holds the spider gears in the third member may have slipped and is hitting the
pinion gear. if I remember right ford used the Dana rear ends for years.

 

It is the drive shaft that keeps breaking. I caught it before it broke this time and only have a mangled u joint. I guess I'll put it on the lift and see if the wheels are still spinning free. If so, I'll crack open the pumpkin and take a look at that pin. I won't have time till Friday so I'll get back to you. Thanks for the input.