Bad MAP sensor can cause the idleing issue,.. as can vacuum leaks. But the tach not reading below 2300 RPM would lead me to seek out a bad ground on the coil or possibly the module on the distributor going bad.

4x2 and 4x4 should use the same computer. Both are air density motors. The only difference in computers for the trucks that I can think of would be 351 vs 302. But either should run the truck fine. If the computer went bad it shouldnt be running at all. The ECM relay needs to trigger fuel pump relay. If bad,.. the fuel pump wont come on.

Hurst