I had the same setup in my '93 (standard transmission) and it often ran cool in the winter without a load. I replaced the (single) thermostat and there was no difference. I suspected back then that it was at least partially because there was no cooling load associated with the transmission. Fortunately, also, I didn't have any overheating problems even when towing a loaded horse trailer in the summer. I had a 3.42 rear axle, so OD was out sometimes, but the mileage unloaded as impressive.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510