I recently had my 2016 Tacoma serviced by the local dealership for factory-req’d 30,000 mi service and they tried to sell me on front/rear diff and xfer case fluid changes at close to $750. The maint manual states to inspect these at every service and replace in severe driving conditions such as dirt/gravel/towing (mine is daily highway driver), but never “req’d” replacement in normal driving. My question, is when is this service really needed and how can you tell yourself when these need changed?
No, go by requirements in your manual. This is just an attempt by your dealership to increase their profits.
If synthetic gear oil was installed when the car was built, I change them at 1/2 the manufacturer’s recommended mileage. If they used and recommend regular gear oil, I change them immediately to synthetic when I buy the car used. That’s how I tell.
That said, $750 seems a bit pricey. I’d take it to an independent shop and ask what they’d charge. If you have it done outside of the dealer, be sure to keep your receipt as proof.