Wiring harness would be suspicious based on my experience with my 92 w250, my 93 w150, and my brothers 92 dakota. The first 2 had severe corrosion at the common splice for the crank sensor and o2. This splice sits right above the transmission and is wrapped and not visible. Every single harness I've taken apart from this vintage has needed it cut out and repaired/soldered.
Your mechanics findings would suggest other wiring issues, but my point is that next step is to take the engine harness out and untape it and inspect every joint and splice.