I'm getting a humming vibration under hard acceleration. It doesnt do it every time or under all circumstances. It does sound like drive train but I could be wrong. I can feel it in the steering wheel, floor board, petals, ect.

This is when I can repeat it, and I know its going to happen:
1. Happens during a highway merger, petal to the metal at about 30-40 mph. This was the 1st time I heard it.
2. Happens at parkinglot speed with heavy acceleration. I almost got hit, so I accelerated somewhat hard to avoid the idiot.

It does it other times, but its really short due to me letting off of it a little bit. I also dont get it when I accelerate from a dead stop.

I have checked the t-case fluid, All u-joints appear to be good. (I grab the drive shaft, and I have no movement whatsoever) Is there a better way to test this?

Any ideas?

 

Drop the drive shaft and see if any of the joints are binding.

Also check and see if the snap ring the holds the rear output shaft in the T-case in place has abandoned ship.

 

My dad's truck had a similar problem, turned out that both u-joints on the rear driveshaft had dried out and were binding really bad.