hey, I got to thinking about it, you could very easily modify a fill plug to accept a temperature probe. There are quite a few that are Bluetooth capable. Temp could be monitored with phone.

 

I have a 1/8” port on the cover for a sensor. Point me in the right direction for a Bluetooth unit, I’ve yet to see this.

 

I was hoping to start with temps that all matched the ambient temperature, but after waiting 5 hours there was still some residual heat in the engine and transmission and I was tired of waiting. The ambient temp was 70°F tonight and after driving 75miles and checking my temps roughly every 15 miles I got the results posted below. During this drive my truck was completely unloaded and being driven on mostly flat ground so I would say the temps posted below are close to the best case scenario for 70°F ambient temps.

It's worth mentioning that my truck has a 170° thermostat installed and a desired ETC of 180°F, nothing has been done to the transmission. My rear diff has a stock cover with a magnetic drain plug added to it, and it is filled with Redline 75W140NS synthetic gear oil and XL-3 friction modifier. All rear axle temp measurements were taken with an IR temp gun and the readings are from the lower half of the diff cover.

Mile 0:
Rear axle cover: 70°F
ECT: 118°F
ATF: 100°F

Mile 15: 70-75mph on the highway in 10th gear
Rear axle cover: 124°F
ECT: 180°F
ATF: 164°F

Mile 30: 70-75mph on the highway in 10th gear
Rear axle cover: 137°F
ECT: 172°F
ATF: 177°F

Mile 45: 70-75mph on the highway in 10th gear
Rear axle cover: 148°F
ECT: 180°F
ATF: 188°F

Mile 60: 70-75mph on the highway in 10th gear
Rear axle cover: 148°F
ECT: 167°F
ATF: 201°F

Mile 75: spirited stop and go driving
Rear axle cover: 149°F
ECT: 185°F
ATF: 210°F