That is a fairly tough one. The obvious swap is the 42RLE which was the factory option but that won't be easy. You will need the underhood harness, TCM harness, and the dash harness to get the over drive off switch. Not really easy parts to find and identify as being correct.
Next option would be the AW-4 but right now there is no good way to swap one in and maintain the cruise control function. (we are currently working on a solution for that)
Last option would be the 32RH which would be the easiest but it is a 3 speed and since you have OD in your trans now, I don't believe you would like it.
If I were doing it, I would acquire a full salvage 03 with the 42RLE, use the parts to do the swap and then sell off the remainder to recover some costs.