I have a 4.0 in a 97 AWD with 180000 miles. Up til now it has run like a champ.Checkengine light came on last week......but still ran fine. Today developed a rough idle and some stumbling/surging on easy acceleration. On wide open throttle it seems to still pull strong and smooth. Any suggestions where to start? Thanks

 

Check the MAF (mass air flow) sensor for dirt on the little wire. It can give you a stumble on acceleration. Make sure the flex host from the MAF to the throttle body is not loose or cracked. It would add 'extra' air that the MAF didn't know about, and thus give you a crumby idle...
Check your fuel pressure regulator. A gauge will cost about $30 at an auto parts store. It can jam and give you sky high pressure... bad idle and sooty exhaust.
tom

 

While you're under the hood, remove and clean the Idle Air Control (IAC) valve located on the front/side of the throttle body. This a cylindrical affair about the size of a silver dollar in diamter and 6 or so inches long. Held by two screws. Remove and clean the pintle pin with carb cleaner. Often, this will gum up and affect the idle air mixture. Cleaning is usually all that is required to restore full function.

 

In addition to what the others have said, it's worth checking/replacing the PCV valve. It's cheap and if failed could cause a bad idle, but probably not a likely culprit. Also, if you haven't done plugs and wires for a long time, now might be the time.

 

Hate to add to the list, but could also be a bad EGR valve... failing to seat at idle.
Pull the codes... it will tell you where to start.
KenP