Joe,
What controls the trans shifting, throttle pedal or shift lever on column? If shift lever on steer column, hold shift lever with a little pressure like you are trying to engage and see if it will stay in gear. Make sure to test in reverse and forward. If it stays in gear, shifter is worn. Also, take off the steer column covers and check all wiring for any wear or damage. Three different models of truck, serial number would help.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.