Again, likely just the programming as its in a different vehicle, it's prbably 99% compatable and would work just fine if used as the engine and trans inputs and outputs are the same. Although the best bet is to always use the same year and model. The m/t PCM just lacks the automatic trans control pins.
96-99 use different connectors for auto ecu and manual ecu. So let's take a 96 4runner. A 96 4runner with factory auto tranny will have completely different ecu/connectors then a 96 manual 4runner. So the engine harness on the 96 auto will have different connectors then the 96 manual 4runner harness. And on top of that, the pin layout is completely different. I know this because I've asked many guys at yotatech who have done 3.4L + manual tranny swap. If you are going from auto to manual and don't want engine code, you can keep your auto engine harness, purchase the correct connectors and repin the engine harness to those connectors for that ecu. The layout of the pins on auto vs manual are completely different. But the wire colors are identical. Now 00-02 its plug and play. I compared the wiring diagrams of auto And manual and its different pin layout
http://www.toyota-4runner.org/3rd-gen-t4rs/180875-repinning-auto-harness-use-manual-ecm.html