i watch for coupons and take mine to one of the quick-lube shops every 5k miles. i know they check the diff cause they bring me a sample of the old on a white plate to show me how bad it is and try to upsell me on a gasket and fluid change. i just tell them no thanks, just top it off. don't forget to check the transfer case too. as you said, Chris, dipsticks don't tell the whole story on fluids.
i haven't changed my own oil again since that incident where the load leveler's siamese cat fell into the the drain pan and ran all around the garage shaking off oil and leaving footprints everywhere. well worth the $20 not to ever have to scrub a cat down with dish soap again.