I'm having the same exact problem. After you slow down and the vibration stops, does your truck make a really strange sound if you turn to the right?
I've had over $1,600.00 of work done to try to find the problem, so far I've got new front end u-joints, wheel bearings, pinion bearing in the rear axle (mechanic thought the noise was traveling up the drive shaft).
The noise went away for the whole summer, but just the other day it started back up again. It doesn't seem to affect the drivability at all. And will come and go with the bumps in the road. By this I mean that if the vibration and noise starts, it will stop and start again as the front of the truck slightly bobs up and down with the slight bumps in the road. This is what makes me think it is something in the front end and not the torque converter.
One other thing, don't know if this means anything, but after one session of the vibration I crawled under my front end to take a look. I noticed that the area where the shift motor is in the front axle was very warm, not exactly hot, but realy warm. I didn't have the 4wd engaged at all, so I can't figure out why that area of the axle would be so warm.
Man, I hope someone out there has an answer for you, cause this damn problem has been driving me nuts. I'm just about ready to scrap the whole front axle and replace everything.
Good luck.