Driveline vibrations

Damnit! It's been asked and answered so many times, but I can't remember...

Driving on the highway at 60mph (ish) slight vibrations goes away when I let off the throttle. That's a drive shaft u-joint? But can you tell which from the symptoms?

Could it be transfer case related?

I'd think you'd be able to shift the shafts and which ever U-joint shifts either side to side or back and forth would be your bad one.. or wearing out one.. At least that's how I found the bad one on my old drive shaft..