My 2007 Yukon 4 WD with 3.73 gears and 60,700 miles was just in the dealer's for a rear end rebuild. Started hearing some noise from rear end a few weeks ago. Had tires rotated but didn't help. I checked rear end fluid and it was full, but there was a LOT of filings on the end of the magnetic plug - didn't look right. Took to dealer for a check. They pulled apart and said rear end "was toast." Wanted to know if I pulled heavy loads. I pull a Pace box trailer (from Home Depot) occasionally - probably a total of 5,000 miles - with relatively light loads. Tech said the weight wasn't the issue on that trailer, but the high flat face of the front of the trailer offered a high wind load. He's right - noticed that on a trip from Texas in a head wind, but had slowed down to 60 mph. They rebuilt rear end - ring, pinion, bearings, seals - under the drivetrain warranty. Tech said they are now using 75w-140 synthetic in the front and rear diffs instead of the stock 75w-90. Guess they've seen some wear when using the stock 75w-90 fluid. Glad I had the 5 year/100,000 mile powertrain warranty.