Rumbling noise when accelerating?

Check the center bearing on the driveshaft for play. Also with the wheels chocked, check the drive shaft (twisting action) for spider wear.
You missed something somewhere, or the check was done with brakes applied and drive shaft was getting torqued from being in gear during test

 

Ck the U/joints before you grease them for any up and down or any movement in them, replace them if you find bad ones?

 

My understanding is that drivelines are to be checked with engine out of gear and brakes off. Wheels chocked so the vehicle cant roll.
There shouldnt be any play. Check by trying to twist the driveshaft, trying to slide it toward the fron and to the rear, try pushing up (as gravity is already pulling it down), and lastly by trying to move it sideways. Any movement is a sign that you will get vibration. New components wont move (wont cause vibration). The only other thing would require going to a shop that specializes in balancing drive shaft. An out of balance drive shaft will create vibrations as well.

 

Also check your carrier bearing, if you have one on that rig.
I lay on my back, on cardboard, 90 degrees to truck,
and take both legs and push up near carrier bearing; the bearing can be good,
but the rubber body pounded out - in where U can't see it.
The bearing won't spin around in it, but it jumps all around.
If it moves too much, you found your vibration.

What you describe sounds like universals, tho, especially the under load part.

 

I think they got it.
As long as you’re down there. Just an extra. Look where the yolks for the driveshaft go into the tranny and rear end. Check for leaking seals.

 

If air ride, check the height