Not to derail things, but:
Unless I'm recalling some research incorrectly, you can install an early 4L60E with the correct adapter, but you have to install a stand-alone transmission controller in order to make it function...and that's expensive. Like, $1000 of expensive...but I'm pretty sure Novak carries the required widgets if you want to pony up for them. Likewise, I think you can install the earlier 700R4, which requires no controller, but - again - it still requires adapters. Then, you get to have fun with the T-case, but there are some ways to handle that, too.
Thus endeth the derail.
Unfortunately you can't install a transmission such as the GM 4L60E for instance. It's computer controlled just like the 42RLE, and the Jeep PCM has no way of talking to it and telling it when to shift, etc.
Unless I'm recalling some research incorrectly, you can install an early 4L60E with the correct adapter, but you have to install a stand-alone transmission controller in order to make it function...and that's expensive. Like, $1000 of expensive...but I'm pretty sure Novak carries the required widgets if you want to pony up for them. Likewise, I think you can install the earlier 700R4, which requires no controller, but - again - it still requires adapters. Then, you get to have fun with the T-case, but there are some ways to handle that, too.
Thus endeth the derail.