The seal where the driveshaft yoke goes into the pumpkin is leaking. What are the tricks to replacing it? If I remove the yoke does it mean I have to relash the gears? Or can I just mark the nut and then retorque to specs?

Pinion Seal.
Remove 4 bolts on Driveshaft. Unstake nut. Impact off the nut. Pull flange, remove seal. replace. The trick is torqing down the nut. You can't change the pinion depth because the shims are between the pinion gear and the deep bearing. You can't change the backlash because that is in the side/side adjustment rings on the diff. You can change the bearing preload on the pinion if you change the torque on the flange nut! You have a crush sleeve in that diff. It is already crushed and you can't crush it 2x, so you are kinda on your own on the torque setting. Maybe just put it back where you found it? It'll be hard to check the preload with the axles in there. Don't worry too much, this seal gets replaced this-a-way all the time.

ditto what lennies said. Did mine the same way when I regeared. That was thousands of miles ago and still good to go.

Ditto the guys; just make sure you mark the flange so you put the driveline back together in phase.