I can't tell you how much harm is being done, but low oil pressure is never a good thing. It would be a pricey repair. I know gallery cover gaskets are available for the 3.5L, which have the same problem, but I'm not sure if they are available for the 4.0L, yet. If so, it would be a lot cheaper for the gaskets than it would be for the rear timing cover, obviously. Replacing the timing set on these vehicles will usually run around $1600, give or take and depending on where you take it to, so I would expect the rear cover would add several hundred dollars onto that job, if the timing set is replaced, as well, while apart.