i recently replaced my rear u joint which was really worn out and causing a bad vibration. drove the truck for 2 weeks and the vibrations came back. i assumed that the u joint went again due to having the same vibration and humming noise in the rear end. brought the truck in the garage today and the u joints are solid but the drive shaft has up and down play at the boot. not something that was noticable when i replaced the u joint. is the driveshaft no good? or is there a bearing inside that can be replaced?

 

That should be your slip joint. It has a spline drive in it. If it has up and down movement, I don't think there is any repair for it. You will need a need driveshaft.

 

Take off boot clamp and stuff slip joint with a lot of good grease, these are know to run bone dry ,I would try that before buying drive shaft