Originally Posted by Mike Sal
The 4.0 will definitely have noisey lifters with that mileage. Nothing to get to excited about it as long as you know ahead of time that they are going to rattle. As mentioned above, all of the bearings will be suspect. Depending on how the car was used, the door hinges may be sagging. I've found that the seat belt buckles get harder to unlatch when they get a lot of miles on them.
It will also likely have a leaking rear main seal if not already replaced.
Before checking, clean engine thoroughly, especially the back of the block.
Then after miles, if you see oil underneath, run hand/rag along back of block to check for new oil leak. If you see it, the valve cover gasket or the cover needs to be replaced before you can tell whether rear seal is leaking.
A tight 4.0 does not leak, although there is folklore that they all do.
Another feature is that the 4.0 needs high revs every now and then to rotate the valves properly. Seems it won't do it until it is at an rpm that will break all speed limits in normal gears.
A good Seafoam treatment would not be a bad idea.
The crank position sensor may be getting old...
Check for even the slightest sign of front vibration at speed--it can turn into death wobble. Replacing the steering stabilizer might quiet it, but the real cause is always worn front suspension parts.
300K of reasonable miles and treatment isn't a red flag, but it is somewhat on the high side. Replacement engines are all over the place, where even strokers aren't that expensive.
The 4.0 can eat catalysts... particularly aftermarket one size fits all.