EXTREME shaking at 40mph

[WTCBST]

ok so i just rotated my tires front to back and the next day im driving to school and at about 40-45mph my truck starts to shake fucken bad!!! i pulled off check all my wheels, they were tight. so i look at my rear end and notice my u joint going out. i replace the u joint and think to myself why would it go out ? i push my truck forward and notice the pinion angle changing. all the u bolts were loose. i tighten them up and fix the pinion angle. go test drive and it still shakes like a motherfucker at 45. i swap tires back to were i had them and it still shakes...got any ideas?

 

[Loanranger]

Well first off, are you sure you only had one bad u-joint? If your u bolts were loose enough to let the axle move around under there, it's possible it tore up all the u-joints, or possible bent the d-shaft? Also, when you changed the U-joint out, did you make entirely sure that the u-joints were still in phase? That would cause the shake you are describing. If the front and rear u-joints aren't in phase, you will get quite a wobble at around 45-50.