What is a flex plate in a 1991 Jeep Cherokee?

A "flexplate" is the part that replaces the flywheel on vehicles with automatic transmission. Manual transmissions need a clutch assembly to transmit power from the engine crankshaft, to the transmission. That clutch assembly is mounted to the flywheel. A flywheel is a thick, heavy steel plate that can dissipate the heat generated by the operation of the clutch. It also generates inertia which smoothes engine rpms at lower speeds, and helps prevent stalling of the engine while accelerating from a stop. Since vehicles that have automatic transmissions do not need a clutch, neither then do they need a flywheel. They do however still need a mechanical coupling between the engine crankshaft and the transmission. The "flexplate" is that mechanical coupling. Because there is no clutch heat to be dissapated, the flexplate is thin, and generally has holes in it for purposes of lightening the plate, thereby reducing inertia. Because automatic transmissions have hydraulic pumps and internal clutches, to take the place of clutch slippage during acceleration, the need for inertia is eliminated. The flexplate also has the teeth for engagement of the starter motor, usually a hardended ring of teeth, welded to the outer perimeter of the flexplate. If you are familiar with the term "torque converter", and understand it's function, then you will know that the torque converter is mounted to the rear face of the flexplate. It all has to do with power transmission to the transmission itself. It is rather common for a flexplate to need replacing after many miles. They are prone to cracking around the center hole or mounting bolt holes, (they are mounted on the rear face of the engine crankshaft) or the starter ring teeth may be badly worn. Replacement of a flexplate is similar to clutch replacement, and involves removing, or at least moving the transmission rearward, to access the clutch/flexplate area. Hope this makes sense to you, and helps with your problem.