I do know it's all electronic shift, and though you may be in park, the pcm is looking for input resistances from the solenoids and also what gear it's in to make it's idle speed decisions. Also there is a thermistor which changes the shift points for given temperature ranges sensing the temperature on one of the shift solenoids, I think it's the overdirve or rather the torque coverter lockup one.

 

Another update..

Turns out there is a transmission fuse and even though it was making my idle jump up when the TCU was plugged in, I guess the TCU was not actually getting power because there was no fuse in the spot.

Put a fuse in and it drops down to first when you put it in drive now.

Now, the bad news, is even though I have drove it up and down the road 3 or 4 times testing out the transmission, after I test drove it when I discovered this fuse, it died on me, but would start back up until you gave it gas, like it was out of fuel....I put 5 gallons in it, and it is doing the same thing...I was able to rev it up and drop it in drive to get it to the driveway and parked in front of the barn...Not sure what that is about...tried unplugging the tcu to make sure that wasn't causing it and still the same thing...its like 13 degrees outside and this jeep has no windshield and no back window...it was frieking cold, so I didn't mess around with it too much...I had already came inside for the night anyway.

 

This thing is going to be the death of me....I noticed that I have a ticking coming from the bellhousing area....going to pull dust cover off tonight or tomorrow and recheck the torque converter bolts.

Last night I put trans fluid in transfercase, bypassed the neutral safety switch, pulled the check engine codes, change the engine oil + filter, wired in my led light bar switch, fixed the hood, and traced down a blown blower motor fuse, and picked up some of my tools and cleaned up all the oil...wasn't super productive but got some things knocked out of the way.