revving/idling problem

I purchased what I thought was going to be a cheap tow vehicle about 2 years ago. It is a 1989 Jeep Cherokee 4X4 with a (4.0 Litre) fuel-injected, in-line, 6 cylinder engine with 187,000 miles or so on the odometer. After replacing both upper and lower ball joints on both drvr and psgr sides...