Rough idle

I recently inherited an 88XJ 4.0 from my aunt which I am in the process of getting ready for the rubicon. At Idle the engine seems to run a bit rough...almost rythmic if that makes and sense. While my aunt still had it she brought it into the shop and they told her it was caused by one of the main bearings starting to wear out. Does this make any sense at all? The engine does not stumble or anything under load etc. so it doesnt feel like a fuel or ignition issue. Thanks guys

How is oil pressure? Should have a minimum of 13 psi at hot idle and between 37-75 psi over 1600 rpms.

Always review tuneup status when symptomatic. That includes plugs, plug wires, distributor cap, rotor. Toss a bottle of Techron or another high quality fuel injector cleaner in about 3/4 of a tank of gas for concentration to help optimize fuel injector spray pattern.

Cleaning the throttle body and idle air control is NEVER a bad idea. They get carboned up over time and miles.

As for internal condition of engine, running a compression test will give you a pretty good snapshot of how the engine is doing. Looking for 120-150 psi on each cylinder with no more than a 30 psi variation between cylinders.