I second drain and change. I have 4WD so I did the front diff, rear diff and transfer case. And I did a complete flush of the transmission which is more controversial but worked out just fine for me at ~135k miles (it's been at least 15k without issues including towing camper/boat for 1k+ miles). It was all straight forward for the most part except the gear oil for the rear diff was thicker than I expected and a bit of a hassle. I wish I'd had a little extra as you need almost exactly the amount that comes in convenient sizes and who wants most of an extra quart of gear oil sitting around? So if you haven't worked with that stuff before, might be worth reading up on tips. I think one might be to put the bottles into a bucket of hot water so the oil flows more easily. But I dunno.