Two problems related to differential unit have been reported for the 1999 GMC Sierra. The most recently reported issues are listed below.
Gas tank gauge works once in a while. Our mechanic said it was probably due to moisture. Also, the rear differential cover is either cracking or rusting through. I put a new gasket on it and the next day it was leaking again but not at the gasket. About 1/3 up from the bottom, you can see where fluid is leaking through the cover. I called 2 dealers (1 Chevrolet, 1 GMC). They both gave me numbers to other dealers (1 in PA. , 1 in IA. , 1 in upper mi. ). The two out of state dealers told me that the part was for a customer and would not sell it. The one in mi. Also said that, in addition to "that must be a popular part, because I've had every dealer in the country call me. " is this part a defective piece?.
The rear rotors u-joint, carbon build up on throttle, replaced window motor uses 2 qts of oil every 400 to 500 miles , transmission uses 1 qt of oil every 20000 miles , air conditioner fan does not work on high I believe the throttle build up is a safety issue with the gas petal sticking. We have a lemon.