I just did this swap a few weeks ago. Search my username because there are a few good links in the thread I started like to Stu's site. He did it on a TJ, but it's very similar.
You don't have anything to hook into the ECU.
It was a lot easier having the donor XJ for all the little
cr@p. Like I was able to take the brake peddle assy with both switches and swap the whole assy. I swapped into an 89 from a 90. It was not quite "plug n play" although by taking my time and opening up the wiring harness to follow the wiring, it was doable for me and I'm no mechanic by any streach of the imagination.
On my 89, all the required wiring for switches and what not was under the dash. The unused wiring (3 wires for switches I think) were hooked into the same connector unused. It didn't match the connector set up of the donor, so I just changed the connectors. Minor soldering is necessary (for better wire connection, but I guess you could use but splices if not).
You will also need to change the radiator for cooling the tranny fluid, or what I did was mount an external cooler with an electric fan. If you go this route, get one that is thermostatically controlled so it comes on and goes off at designated temps. I wired mine into the fuse box to come on with the key in the acc. position because I did not think to buy this type as Stu suggested. I will find a temp switch to add into the fluid line on mine to control this a little better.
All in all it was not to difficult. I just took my time and asked a hundred questions as I know next to nothing about vehicle wiring and computers. Being confident in getting the wiring correct was the hardest part for me.
