The 4.0 is happiest with the standard Champion plugs, and the cheapest place for them is a Jeep dealer. The same for the oil filter too.
:-D
If the yellow part time light is on it's stuck in 4wd with the centre diff locked even if the lever is in 2wd position. You need to get it out before you break something.
Rear springs are done, pretty saggy, where's best for them, scrapped TD?