Does this bucking etc happen shortly (2-3mins) after starting the engine and lasts only ~ 2mins? This might be due to an aging upstream (not the downstream) O2 sensor. It's easy to determine if it's indeed caused by the O2 sensor, just disconnect it. The connector is attached to one of the engine oil pan studs on the driver's side, approximately above the axle. The 4.0 engines seem to be effected quite often by this issue (ZJs and WJs). With the O2 disconnected you WILL get a Check Engine Light. Don't worry that won't cause any harm, just don't plan any 1000mi road trip with a disconnected O2 sensor. A few days of driving like that won't hurt anything. If the bucking goes away, you diagnosed a failing (aging) O2 sensor. Replace with MOPAR only (not Bosch!!!) or you'll have the same issue with a new one!!! Let us know if this solves your issue.
1997 JGC Laredo 4.0 4x4 99900mi * 0W30 German Castrol since 75k * 225/75/R15 Firestone Destination AT on Ravine rims * winter 235/75/R15 Bridgestone Blizzak DM-V1 * UC suspension * Monroe Reflex Monotube shocks * factory skids * KOR steering brace * rear Addco SB * Magnaflow cat and cat back system * complete 5.9 interior