hey I have 123,000 on my 02 duramax...Should I replace the fluids in the front and rear differential cases? The dealer said he would replace with Synthetics, the shade tree mechanic i know said it wasn't necessary. I pull trailers and off road occasionally engaging 4wd...So what do yall reccommend?

If they have never been changed, it would be good to change them....Do you need to go to synthetics...no, that is up to you. Although, GM put sythetic "grape juice" in the rear end from the factory. I would at least put a good GL-5 rated gear lube in the rear.