AWESOME NEWS! FINALLY!!! lol. Break-in: follow the instructions in the Yukon booklet. It was something similar to as you describe....drive for a bit, let her cool for a few hours, drive for an hour, cool down over night....all this while taking it really easy on her. The theory is new gears under break-in are HOT gears that build up considerable heat. And heat makes gears brittle and they are killed. Then fluid changes after 500 mi or so. Plus that booklet is a neat little read and I keep mine in my owner's manual case. Fluids: I'd refer to what OX approves/suggests for fluid. I can't remember if you did anything up front carrier-wise. My locker, an Auburn Ected, requires dino oil plus a L/S additive. Apparently running synthetic or dino with L/S additive built in gives hell to my locker's clutches. Numerous folks have experienced this when running synthetic with my locker, so the Auburn spec is pretty specific and accurate. If OX doesn't suggest anything specific, sure run a synthetic when it's time to swap fluids. I'm a fan of Royal Purple. My gear guy suggests L/S additive in any axle running any oil...he has a theory of stickiness with metal contact, so I've followed his advice. EDIT: Note: I'm coming up on 5 years being geared. Crazy how times flies!