How to Stop or slow leak

Seal conditioners are already in the gear lube. The pinion shaft is up high enough that not all the lube is going to run out, but if the level is low, no lube will run out to the wheel bearings. Replacing the pinion seal is rather straight-forward and not too difficult so there's no reason to put it off. If it's leaking very badly, the bearing might be worn allowing the pinion shaft to move away from the seal. That will be a lot more costly but you should be noticing a howl.

The lube is also used for cooling. It's not unheard of for the differential to turn red hot when the gear lube was very low. Believe me, you would rather live with a pack of skunks than take one whiff of a burned up rear axle! Had one once that cleared the shop in no time!

You can ask at the auto parts stores but I've never heard of what you're looking for that works with gear lube. The best I can suggest is to keep the differential full.

It depends too on how fast it's leaking. A little wetness is nothing to get excited about but if it's leaving drips on the ground, that should not be ignored too long.