Help me out guys before I tear into it. 98 ford Ranger 4x4. Upon DECELERATION only, there is a metalic knocking sound in the drivehshaft. It may be resonating from the rear end. Jacked it up and ran it and does the same thing. U joints are good. Everything one each rear wheel checks good. Any ideas?? thanks

 

If you have the allumium drive shaft. Then the little plastic cap inside the slip yoke has come loose. Just take it apart and remove the cap and then your good to go

 

I'm not positive but I would say it looks like a steel driveshaft. It is original to the truck.

 

wasn't 98 aluminum? there is a thread around here that goes into detail about what years were steel and what were alum.