Long coolant answer
How many miles do you have and what does your Service Guide say regarding intvals? If I remember right, my '97 lists 100K miles for my differentials. It might have even said filled for life unless the differential gets submerged in water. I know it was a very long interval. I think the recommended interval for my coolant is every 3 years / 36K miles. You are smarter than a lot of people that take their car into a dealer. They usually figure the dealer knows best so what they are telling them must be true. The dealer does not always know best unless you tell them that all of your maintenance has been performed by them and they look up the last time your fluids were changed. And then they only know best if they are being truthful to you. How do they know your differential fluid needs changed, because of mileage? If you just changed your fluid yourself 10K miles ago how would they know that? They wouldn't!
Antifreeze can get dirty and should be changed per the recommended intervals. It is cheap and easy to do yourself if it does need changed. I had a local dealer insist that they could not troubleshoot my overheating problem until they performed a coolant flush since it looked like I had the wrong coolant in my radiator. I had done a coolant flush 3 months earlier and used the Peak Extended life antifreeze. It is orange rather than green. A month later I noticed my waterpump leaking so I had the same dealer replace my waterpump under my extended warranty. I told them I had extended life antifreeze in it and wanted them to put the same antifreeze back in since if they were to add the regular stuff my antifreeze would loose its extended life features (came straight from the bottle). When I picked it up, I quickly glanced under the hood and saw my reservoire was full of the extended life antifreeze since it was still orange. They actually listened I thought. A week or two later I checked the coolant in my radiator to discover that it was full of the green antifreeze. Well 2 months later when I had the overheating problem my orange antifreeze and the green antifreeze had blended together in my reservoire making a brown colored antifreeze. When the Tech saw that he assumed that I had used the Dexcool antifreeze which is not compatible with standard antifreeze. He went so far as to say that my two antifreezes were separating and unless he performed a complete power flush he couldn't track down the overheating problem. He said those two anti-freezes were not to be mixed together. I argued with the service advisor the fact that they were the ones that mixed the two together when they replaced my waterpump 2 months earlier but it didn't matter. His Tech would not look at it again unless I authorised them to do the flush. I told him to go ahead and do it if that is the only way they will work on it. When I called them back later that day they said that they did the power flush and found that the thermostat was stuck closed causing it to overheat. It was ready to be picked up. Expecting hassles regarding the coolant issue, I brought my bottle of extended life antifreeze with me when I went to pick it up. I also mixed a 50/50 solution of the green antifreeze with the orange antifreeze the night before to see if it separated like he said it was. I took that bottle with me. It looked very brown but it was not separating like the Tech said it was. When I picked it up they had a $135 charge for the power flush. I argued that I shouldn't have to pay for the power flush since I had just replaced the coolant 3 months earlier and they replaced my water pump 2 months earlier. Even if the two coolants were not to be mixed (the bottle says otherwise) they were the ones that mixed them together. The Service Advisor wouldn't budge so I went and talked to the Service Manager. When he asked the Tech what actually fixed the problem, the Tech said the thermostat. The Service Manager told the Service Advisor to back the $135 power flush off my bill.
So as you can see, the dealer does not always know best. The best thing you can do is check the supplement that came with your Owner's Guide so you know the truth.
