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Charter Member
Full Member
Jan 18, 2002
Area 54
I had my vehicle in for normal service and the tech noticed that the rear diff was lower than normal on fluid. They topped it off,but also informed me that the service computer said that the fluid should be changed after the first 5000 miles( I am at 14,000). They use synthetics at $16/qt. I didn't have it done yet. It dawned on me this may be the reason the posi felt like it locked up going around in a parking lot last month. Anyone change theirs yet? ???
I think your dealer is misinformed...

1) Scheduled maintenance shows nothing about rear diff fluid changes at 5000 miles (in fact, the first service for the vehicle (except for oil/filter changes) is at 7500 miles).

2) I would also wonder whether they overfilled your rear diff. The owners manual specifies that the lube level should be 5/8" to 1-5/8" BELOW the filler hole (which seems like a major pain to accomplish in real life).

Finally, these trucks don't have positraction, in the traditional limited slip with clutch packs. They have an Eaton G80 locking differential, which provides positive locking or open differential action. From what I understand, the transition from open to locked is accompanied by popping noises - and is normal.

Additional info: looking through the entire manual, there is NEVER a mileage where they recommend replacement of the front or rear axle (diff) fluid - only top-off services. Only the main tranny and transfer case fluid requires changes.

That being said, I think it may be worthwhile to change to synthetic differential fluid, especially if you do towing, etc.

FYI . The dealer service mgr. told me the rear has synthetic fluid in it from the factory.
Yes the Rear End has 75-90 synthetic in it.., and the Front has Dino oil in it.., I have 1800 miles on mine., and Im changing it this weekend.., it never hurts.., and I want to have a look at the Gears., just to make sure.., all is well.., the Service Manager said to use Mobil 75-90 Synthetic.., its the same stuff GM uses., only under there brand name, and its cheaper., and since I had a problem with the Front Differential., ill probably change the Lube on the New one., at 1000 miles.., just to make sure theres nothing strange going on in there., when I checked my old one., just to be sure it was topped up with fluid.., the fluid up there was nice, and clean looking.., but when we drained it at the dealership yesterday..., it had lots of Junk in it., so it never hurts to play safe........