I switched my pcm into another truck for test purposes, but when I reinstalled the pcm back into mine it would not start. I sent the pcm out for testing and it came back fine. I now see my security light on in the dash and my rear defrost light on the switch flashes with the key on. It cranks but will not fire. I can hear the fuel pump come on. Is there a way to reset the security system?
Nope. I never ran into this on a truck but it was a problem on early '90s cars that did not have the factory-installed security system. If yours does not, you will need new Engine and Body Computers. Typically what happens is when you put your Engine Computer into a vehicle that has the security system, that computer learns that programming from the Body Computer. That "upgrade" programming can not be undone. Once you put that computer back into your vehicle, it teaches that upgrade programming to the Body Computer. From then on they're both waiting for the disarm signal to let them work, but that signal is never coming because that system isn't on the vehicle. If you continue to replace one computer at a time you will just keep on having the same problem, and from then on all of those computers will only work on vehicles with the security system. This is a problem too if you buy a used computer from a salvage yard because they don't know if that option was on it. Any vehicle with the security system can use any computer; it will upgrade the programming by itself if necessary. If you don't have that system on the vehicle, that's when you have to install a computer without that programming, and that can be impossible to know unless you buy one from a rebuilder or the dealer.
That was never a problem if your vehicle did have the security system. If your truck does, I would look first for a bent, stretched, or pushed-out terminal in one of the connectors. Don't forget to unlock the front doors with the key to disarm the system.