I have an '86 Jeep Comanche with a leaking pinion seal, I don't remember how long it has been leaking. I thought it was the rear differential gasket and changed that but it still leaks. Hope I can go a little longer. It's been halfway across country once then all the way. It needs a new rear end but it is still holding up. I don't want it to break down on me. To much work to do. Please help.
I assume you mean damage to the differential, not to the environment :-)
If the leak is slow and you keep up with it, you can probably go a long, long time before it is anything more than a nuisance. Slow means that between checks and topping up the oil the oil doesn't drop too low.
I've you've been running it dry, their may be damage already, but heavy gear oils won't drain away quickly and there should be a fair amount of oil below the pinion seal. You are probably OK, at least on this problem.