I would suggest before you throw $100+ at a re-manufactured EEC-IV, you check some basics. The computers are very rugged in your truck. They do not fail very often, and not often for no reason.
Remove the center wire from the distributor, and put a spark plug into the terminal. Ground the outer shell of the plug, and crank the engine over. You should get sparks. You should also get a spark when you turn the key from off to on to off. If you don't get a spark, you have an ignition problem that the EEC-IV module will not likely correct. You can run the ignition system with the timing jumper removed to take the module out of the picture. It will then depend solely on the thick film, the pickup and the coil. You could have a bad coil, and they are about $13.
If you look on this site or rangerstation you can find the instructions to pull the codes using an analog VOM, and a couple jumper wires. I will not search for you.