The constant slamming thru the drivetrain when turning. It was actually difficult to shift in turns because of the locking and unlocking of the differential. It wasn't noticeable when I had the automatic. I didn't know what to expect, I never had a locker before.
Glad you found it. Most people don't realize that driveshaft angle changes under load. At 40 mph the drivetrain is under load just from the wind resistance. That's why it's usually set at about 3 degrees static and moves close to zero while driving. And before someone chimes in, driveshaft angle...
Is the blower motor on when stopped? Yes= It's a vent issue (vacuum and flaps). No=It's a motor or voltage supply issue. Anything temperature related will only affect the compressor operation via the hi/lo switch. Engine temp/fan clutch is irrelevant.