2003 Rubicon

Last couple of weeks I have developed a problem in my drivetrain. I'll be driving at 25 to 35mph on pavement in a straight line and there will be a huge bang that sounds like someone is hitting the underside of the Jeep with a baseball bat. The Jeep will transmit the jolt through every part of the vehicle. I took it to the dealer and they couldn't find any thing wrong with the drive train. It has done it a couple of times since I got it back of course, any thoughts, ideas?