I am very suspicious of it being a transmission issue when you say the noise is coming from the right front end under power. That sounds like a front end or driveline issue to me, and I find it very hard to believe that a dealer- or any technician that's not just a lube tech- would confuse a front end issue with a transmission issue. Then again, stranger things have happened, so...
My questions for you are,
1) How long have you owned the truck?
2) Does the transmission shift normally?
3) What does the transmission fluid look like on the dipstick, what does it smell like, and is it at the proper level (check with it at operating temp, while running, in park)?
4) Have you had the transmission serviced in the past 60,000 miles?
5) Have you recently had any front end service done, such as brakes, tire rotations, shock replacement, or anything at all?
6) Have you jacked up the front right corner and actually looked around to see if anything is loose, such as wheel bearings?
7) Is there any vibration associated with the rumbling, and where do you feel it- floorboard (passenger or driver or both, or in the steering wheel)?
8) With the truck on all four wheels, have you had someone wiggle the steering wheel left and right ever so slightly while you feel for any movement anywhere, such as between the inside of the wheel and the dust shield?
9) When was the last time you greased the front drive shaft u-joints?
10) What prompted you to take it to get looked at?
