The short answer is that you can get an aftermarket ECU for the 4L60e if you want one, but the decent ones are close to a grand. The stock truck used the same ECU for both engine and transmission control so trying to use it with your older engine would be an exercise in frustration.
The newer transfer case won't do you any good because it is a driver drop and your truck (assuming stock) is a passenger drop.
Depending on what engine is in the donor truck, you might be able to pay for the standalone controller by selling the engine/TC out of it. If so, that could be a decent choice.
Better yet, just take the whole engine/trans combo out and LS swap your truck. Then you don't need the standalone transmission controller.
Another thing to consider in this process is that if you are switching to an automatic transmission, you will need a transmission fluid cooler to go with it. How large of one will depend on what trans you get and how you set up your torque converter.
