2008 F250 4x4 Drivetrain Popping noise

[divmat]

I have a popping noise in the drivetrain. It normally only happens on take off, but not all the time. I have had it into the dealer several times, but they
have checked all areas of concern and can't find anything wrong. It sounds like a u-joint popping or binding but I don't think that's the problem. It happend once for the mechanic, but he just couldn't find anything. I think it's coming from just under the front cab portion of the vehicle. I thought maybe it was a tranny or transfer case mount loose and popping, but that's been inspected as well. Also thought it was the locking mechinisim inside the tranny or something. I can put it in neutral for a few minutes, then back into drive, and it will pop as I am accelerating but does not happen all the time.
I now have about 84K on the truck and seems it's happening more, but it's been doing it since warranty miles.

There was a problem with the coupling sort of clunking on take off, but they keep that from happening by filling it with a special new silicone grease.
That's not the popping noise I am refering to.

I searched the files, but no luck. Any suggestions for my dealership to check please ?

Thanks
Steve