GMC Trans

I went out to the garage today to take my 1991 gmc 3500 4x4 up to yhe car wash and no reverse??? checked the fluid it was fine. Decided I would take it up for the wash drive was still working. On the way there I notice no overdrive?? I have the 4L80 E trans Does this sound like a mechanical problem or electrical? THANKS

I would start checking the electrical plug-ins to the trans.
Look for dirty connections, broken wires. My sons just had a simular problem and one wire rotted out of the harness

I had a problem like this once with one of my older GMC 4x4 trucks. Turned out to be a small part (seal) in the trans that failed. Only trouble was the trans had to be completely disassembled to replace it and might as well rebuild it while its apart.

Same thing, working fine one day and no reverse the next day. Now, I have no way of knowing if this is your problem and suggest you consult a pro. When I tallked to the GM dealer at the time, they could not tell me what it was but a trans shop pro knew exactly what it was. I recall the trans pro said this was usually a result of shifting the trans from reverse to drive and vice-versa before coming to a complete stop, like getting stuck and rocking the truck back and forth to free it.