Run it till it gets worse!?!?

Carrier bearing. I was experiencing the same conditions that you describe. Get under the truck and push up on the drive shaft near the carrier bearing. If there if play/movement, it's probably due to be replaced. I changed mine out along with new u-joints by watching videos of the process. Not a big deal.

 

I think it's more of a tolerance issue with the OEM needle bearing. THe ECGS alternative has tighter clearances ... this video explains it: https://youtu.be/BOvoM5irtEc?si=OPTlOSz_HUmVpk6v

 

Update?

 

I picked it up at the first dealership to bring it to the other dealership for a second opinion. And of course, the vibration was gone. The second dealership had me ride-along with a technician and he couldn't detect any abnormal vibrations. I've been driving it a couple of days now and I still feel a slight vibration at times, but nothing like what I felt that compelled me to bring it in in the first place. I don't think I'm crazy. It was bad enough that I was afraid I'd have to get it flat bed towed. The tech at the second dealership said that it was possible that the slip joint was binding and that it freed up. Sounded like a reasonable explanation. Im going to keep an eye on it and be sure to grease the heck out of the one greasable Ujoint and splines of the slip joint.