Just making sure we were on the same page... The computer is unresponsive.
This is caused 99% of the time by a shorted sensor. The three big culprits are;
crankshaft position sensor (on the back of the block, just behind passenger side cylinder head)
camshaft position sensor (in distributor)
vehicle speed sensor (in tailshaft on a 2x4, or transfer case output housing on a 4x4).
If any of these 3 sensors get's shorted out, it will put the controller to 'sleep' for a lack of a better term. Unplug them one at a time, then cycle the key and watch for the check engine light to come on for bulb check and the fuel pump to run for a few seconds.
When you see the check engine come on for bulb check / hear the fuel pump run you will know you've found the shorted sensor, replace it.
It's possible you have a power or ground issue to the PCM, but 99% of the time it's one of the three sensors above shorted out. Depending on how the truck was built it may or may not have the vehicle speed sensor, I believe yours will though.
The reason I asked about a recent transmission replacement is that often times the crank sensor wire gets pinched between the bellhousing and block, then 200 miles later when it finally wears through the insualtion the truck quits + acts just as you describe.
Check it out, let me know what you find. It's most commonly the crank sensor. If this information helps you out please click accept, if it doesn't reply back and we'll move on to the next step.