Toyota Bench press
Yep, I think you are onto something, RedChilli. I am running only el cheapo GL-5 conventional 75/90weight oil with a tube of BG gear concentrate additive.
Considering the low price I paid for the yukons, I bet they are the "economy" yukons too.
Talked to a good ol' wrench turner here at Autoaves and he asked the same question. "What weight oil you runnin'? Synthetic? Ford is now specin' 85/140 synthetic." He knows it is a toyota, but listened to my problem and right away thought wrong oil.
The sound is also barely audible at 53mph. It is probably just pinion bearings and most people would probably never hear it or think it was just a 43mph wind whistle. (That is what it sounds like really.).
I just got back from Boulder. 10 miles in traffic and then about 20 miles at 55-60 MPH. Yeah, the diff was hot when I stoppped, but again, I can keep my hand on it for about 5 seconds. My transmission shifter gets almost that hot! No smells, no smoke, no leaks, no other bad noises. I think it will be OK and will probably quiet down when I change the oil at 500 miles and switch to 85/140 and maybe even synthetic at that point. 5.29 is a pretty steep gear. I'm sure it generates a bit more heat than a 4.10. We'll know if I did something wrong on the next trail ride. or the next 200 mile highway run!