What if the rear differential is leaking what.

Depends what's leaking. Front pinion seal requires removal of the drive shaft and yoke at a minimum. If the bearing is worn allowing the pinion gear to move away from the seal, a new seal alone won't solve that. That would require new bearings and total disassembly to get the pinion gear out. That bearing would be making horrendous noise if it was bad.

Axle seals require removal of the rear cover, removal of a small bolt that often shears off, removal of the large shaft, clip on the end of the axle shaft, removal of the brake drum so the axle shaft can be pulled out. Almost all mechanics will do both axle seals right away because it takes a long time to do one, and only a few minutes more to do both.