Yesterday, towing about a 7500lb trailer (not heavy really LOL) in T/H mode, when the tranny would upshift and lock the TC (around 70-80 km/h, about 14-1500rpm) the truck developed a really loud, heavy harmonic vibration from somewhere in the DT...

When the speed/rpm increased to around 100/1700, it smoothed out, but it was **** loud at the lower levels.

It's an 06 3500 DRW, bone stock with the 48re. FWIW it was -26 degrees C, but the truck was warm, and had been driving for hours. The vibration didn't matter whether it was in 2 or 4wd (hell of a blizzard near Wandering River last night), and it wasn't the usual low vibe from the transfer case.

Any ideas? Maybe just a harmonic from the engine rpms and load transferring through the DT, or should I be looking for something?