Trex -- Positraction is just a brand name for one of the limited slip differentials put in the rear end of cars. I'm not that familiar with the lineup on Grand Cherokees, but I suspect that, like my T4railblazer, an LSD was an option on the new ones. She may not have particularly ordered it, or even wanted it, but if it was in the particular GC she liked, it would have stayed in there when she bought it.

Most LSD's (except gear-driven ones) require that a friction modifier additive be added to the gear oil in a differential. The gear oil should be replaced every 3 or 4 years, depending on the type of driving. Sometimes, just adding another 4 oz. bottle of friction modifier additive to the existing gear oil will do the trick. Also, several types of gear oil available now come with friction modifier already in it.

There are basically two things that can happen if the gear oil ned some friction modifier additive. (1) The LSD clutches
can slip a lot, which means that the LSD wouldn't work as well, and the rear end would act more like an open differential. (2) The LSD clutches can start intermittently grabbing, causing the LSD to repidly engage or disengage. #2 is morst likely to happen when: (a) the car is in a turn, which means that the wheels are traveling at different rpms, or (b) when power is suddenly applied or removed when going straight. Of course, these symptoms would be exacerbated if the road surface is anything other than clean and dry.

If she has the original sale paperwork from the dealer, it may state whether or not it came with an LSD. If she doesn't, then you may be able to take the VIN number to a Jeep dealer and it may show what type of rear diff it has. There's probably some web site you can go to for that decoding info, but I don't know what it is.