Well..let's take a step back. We know excess load on the CVT will make lots of heat..we also know all rear ends also make more heat when they are loaded..as in a loaded truck verses an empty one. So it would make sense that anything that would load the diff would also transfer to the drive system..ie clutches and belt. So..what after the diff can cause this? Or is it the diff itself...or could it be another heat source causing the CVT load but heating the diff.
I recently...and again...took mine for a run down the road and forgot to release the hand brake. It was a bit sluggish but went fine. After about 1/2 mile I started smelling something being toasted. Looked down to see that tiny little hand brake warning light on in the dash. So I stopped and checked it out. Yes..the pads were again toasted but also the disk was red hot...which transferred some of that heat to the pinion and into the diff. The front was noticeably warmer but not overly hot...but then.. it was only 1/2 mile.
OK, what else...these things are well known to have sticking brakes. Many peddles were adjusted wrong with no play at the top which wouldn't allow fluid to release back to the master. That holds the pads against the disks and causes them to wear faster yes, but also makes load on the drive train. Something to check anyway...and not just the rears..fronts dragging will also make you push harder on the gas to make it go..same thing, loading the drive train. Big heavy tires? Yes, but mostly during acceleration. Once at speed the extra load isn't enough to cause much extra heat. How does it roll or coast in neutral? Any wheels not spin freely? How's the play in the rear diff?
You do definitely have something going on. It's just a matter of finding and fixing it.