Hey man, I hope you got the Eaton, because there are rebuild kits for it. But I doubt you would really wear one out fast unless you have a lot of horsepower. The old fart that installed mine said a rebuild kit is about 150 bucks. I expect mine to last at least 150,000 miles or so or I will be pissed.

 

Depending on the posi unit you have you can check the preload on it. Jack one wheel up, use a torque wrench and see how much torque it takes to turn the wheel. Different preloaded lockers have different values here. I know Auburn has a spec sheet on their page.
As for preventative, make sure you are using the specified fluid and friction modifier for your unit.