It's kind of a hard question to answer... a rebuilt transmission is better than a worn out original one. Hopefully they had the work done at a good shop and it wasn't the shadetree ******* tune up type. If they have receipts thats a plus. A good test drive will give you a good indication. Listen for weird noises coming from under the floorboard area, make sure it doesn't pop out of any of the gears and I would check to make sure that the clutch isn't slipping (In second gear, slow down and let the engine just push the Jeep along at idle then floor the gas pedal. If the engine RPM's rev high and then the Jeep's speed starts to climb, the clutch is worn out. If the engine starts to shudder as if it is struggling and the Jeep slowly accelerates, the clutch is good.). I would also make sure the the transfer case works in each drive.