[THEFERMANATOR]

Does the speedo work? Any codes in it? Most likely it's a speed sensor issue with the white sensor in the tail section of the transmission. I'm pretty sure that it has an output speed sensor in the transmission that is just for the PCM to shift gears. It's been awhile since I've worked on one, but I do know that they were problematic in the trucks and would cause exactly what you describe. The sensor didn't use to be expensive either. Also check the trans throttle cable that hooks up at the throttle body to make sure it isn't hung open. You probably have 3 cables, one for the cruise, one for the throttle pedal, and the last one that will have an adjuster on it will be for the trans. If it gets hung open it will not allow it to shift.