No power to wheels while driving.

While driving, something will cut power to the wheels suddenly, motor seems
to still be pushing power when pedal is pressed, but the motor just revs, it
doesn't get to tires.

This happens during two occasions:
1. After driving all day on the highway around 75 to 80 mph, it'll start
happening and generally limit the driving speed to around 65 to 70 mph after
that. Doesn't seem to get much worse after that.
2. While pulling a trailer. That gets real bad. Progresses in the same manner
as above, except driving speed will continue to get more limited as time goes
on. At one point, I couldn't go over 10mph and going over hills was a nogo.

Note: Parking for 30 minutes seemed to recover some of the issue as top
speed became about 40 mph. Quickly degenerated back to 10 mph though.

Note: First time pulling was with a 36' travel trailer. 2nd time was with a small
uhaul trailer. The uhaul trailer event was not as bad, if it matters.

Note: Mechanics thought it was transmission related so a transmission and
transfer case swap was performed. Transfer case needed to be swapped for
other issues. The problem has continued to persist.