car vibrates and/or hums loudly when foots on lights on gas, just cruising, between 40-50mph. if I let up of the gas it stops; if I press harder on the gas it stops.
any ideas?

 

U-joints, loose pinion nut, pinion bearings, extension housing bushing... for starters

 

Losing the weight on the drive shaft might be a cause too.

 

Balance won't come & go w gas pedal application. So, not applicable to this situation.

But, a good thing to look at in general, in vibe situations. And, if found, very easy to verify with a hose clamp; just put one on the DS with the screw where the weight used to be, and see if it's better. If so, add another. If so, add another. And so on, until it starts getting worse again; in which case, turn half of them one way and half the other a little bit, until you again go through the "optimum" dip in the vibe.