Transmission has no drive gear and check engine light came on

Truck listed above is a 3/4 ton with a big block and four-wheel drive. Writing this for my sister. She tows a two-horse trailer. She stopped at a red light and the check engine light came on and it had no drive, no matter what the gearshift position in the forward gears. She shifted it to reverse and it moved. Shifted back into drive and the light went off, forward drive was restored. This occurred more than once. Sometimes shifting to reverse seemed to restore drive, but sometimes not. I have some experience with transmissions and I feel it's an electrical issue because the check engine light is triggered when the fault occurs. Perhaps wiring damage or a solenoid issue? No issues when the fault does not occur. Shifting is normal and her mechanic recently changed the fluid. No slippage at highway speeds, just an apparent failure to come out of neutral. My sister lives a distance away and I'm trying to give her some direction. Any advice would be appreciated.