Just call me Captain Obvious, but it sounds like it was misdiagnosed on the first ECU, and that isn'tthe problem. The second one sounds like it has it's own issues. You're a newbie you say? You want to learn? I recommend you go to mitsubishilinks.com and look/print the fsm for your truck. Check out the " 89 3.0 driveability manual" to your printings. Follow the steps/procedures in those. Another thing you can do is a "haztoys wiring tune-up" google that. Have your white vinegar and water spray bottles handy. While cleaning figure out what all the connectors are, notice their locations.

My $.02.