I am having a strange vibration/sound while under load at any speed.

It sounds like a "bass" speaker (continuous WOOM) and it gets loder if I accelerate hard, when speed is at 60mph and higher is very noticeable.

I can speculate from turbocharger/intake blowing or transmission. It goes away completely if I release throttle and/or shift in neutral, so I assume drivetrain is not responsible (bearings differentials, T case, etc).

I checked engine mounts, and they look ok, do they go bad after a while ? They look like very simple mounts.

I just installed a used transmission/converter, it runs fine with no problem, but whole rig is "under scrutiny" for a while.

 

I had this problem also. It initially sounded like drive train but there was no vibration just a sound. Mine was doing it while pulling my fifth wheel. In my case it turned out that my lift and transfer pump had gone bad. once those were both replaced The problem went away. hope that helps.