FWIW-All 3 of my Dodge trucks seemed to "sheer" the oil quickly. 2000 Dakota, 2004 2500, 2008 Dodge Power Wagon (which i still have). I tried all different types & weights of full synthetic gear lube. It didn't matter. after 10-12,000 miles the rear axle was noisy. Change the fluid & it was instantly gone, but came back in 10-12k miles. Then I added 1 qt of Lucas Lube 140 wt gear oil to the full synth gear oil. Noise stayed gone for 24,000 miles, but i changed it out anyways. Fluid is much cheaper than a new axle. I still change it at around 24-25k.
I used to tow a 30'-10,500 lb toy hauler with the 2004 2500 & the Power Wagon. I have recorded some CRAZY temps on my rear diff cover, with a laser thermometer, when i was towing that camper. 1150 lb empty hitch weight. Highest was 315*F (I changed the diff fluid as soon as I got home). Another time it was 254*F. These were on 95*F days. I have since put a Mag-Hytech diff cover on, which took the fluid cap. from 3 qts to 6 qts. I have not towed far with it on yet. But when i did, I was getting temps of 170*F. (NOTE: We have since traded the toy hauler & got an 18'-3500 lb camper with a 350 lb empty hitch weight). I am thinking the low gears (4.56) in the PW just don't like to do the 70 mph+ speed limits in most states. Maybe I should just slow down, but then risk getting hit from behind on the highways.