Rear diff is sounding like the entire rear end of the car is dropping out whenever rear wheels kick into 4wd. I'm on my second rear diff and the problem still exists. It is a 2001 manual five speed so could it be the front splines for the drive shaft slipping/jumping causing this problem. I'm totally clueless as to why this is such a violent shaking/jumping.

 

welcome to the forum. wow, sounds like something has worn out or broken. when a rear end tooth breaks, it will make a banging noise if the teeth are worn out bad, it will howl. i have not seen worn splines on the drive shaft, jumping sometimes comes from oil on the clutch plate.