2000 Bravada question about changing locking rear axle oil

I have a 2000 Bravada that has the "hopping" problem during slow speed turns such as parking lots, etc. The transfer case fluid has been changed twice in 2.5 years, the last time being a year ago using the
recommended GM fluid. The hopping has gotten worse and a GM dealer told me since I have a locking differential that I should probably change the rear differential oil as well. The owners manual says to only use GM axle lubricant, PN.1052271. There is no description of what that lubricant is. They also give the same part number for the front axle lubricant which is a standard differential and call it "SAE 80W90 ". When I called the dealer they tell me that my book is wrong and that I need this special SAE 75W90 oil at about 29.00 a quart. Now if someone could please answer six questions for me /i would be extremely grateful: 1. What is the correct oil for this axle? 2. How much do I need for a complete drain and refill? 3. Is there a drain plug on these axles or is there a plate on the rear of the carrier that has to come off? 4. How likely is it that a rear differential oil change may cure my problem ? 5. If this is my problem what is actually happening in the differential to cause this? 6. How much damage typically occurs from driving a car with this type of symptom for any length of time and what will we notice? Thanks very much for any advice. Lenny Stein.
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1. I changed gear oil in my 03 Envoy and couple years ago. The p/n was changed from that recommended in the manual to 89021677. It's almost impossible to find the specs on the recommended oil. I emailed Mobil 1 to see if their synthetic oil met GM specs.... answer = no. I have a friend that ended up using Royal Purples gear oil. RP told him it met GM specs. 75W90 is the correct viscosity. 2. You'll need 2 qts. 3. There was a drain plug on the Envoy's axle. You'll have to look..... maybe clean some dirt off. 4. Hard to tell but I recall in some of the forums this was a recommendation 5 & 6 No information.
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I should have paid more attention to the fact it's 2000 model. The gear oil in the 03 is synthetic and about the only place to get it is thru a dealer and it's expensive. I don't know why the oil for yours is that expensive unless they are recommending something newer.... which it sounds like if it's 75W90 and $29/qt. If warranty isn't an issue I'd suggest trying Mobil1 75W90 or RP synthetic.
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wrote:

If it is a GovLoc rear diff it is not fussy about gear oil. It does not need to be SYN, detriot did this to try to squeeze another .1 MPG out of fleet, not because it is needed. ----------------- TheSnoMan.com
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