Tim, unless you are willing to do some fabrication and adapt non Jeep transmissions to your Jeep, you have limited options. The one you have, the AX-15 is considered a pretty good transmission. I have rebuilt one and as far as I am concerned, they are stout enough for the 4.0L six. But if you want one that is an upgrade that bolts in, then the NV3550 is the only bolt in option. I believe they are found in the 2000 and up Jeep XJ and TJs.