suddenly no reverse on my truck

It's not a relay and not likely a solenoid failing in the transmission. It sounds like it's time for a diagnosis from a transmission shop or a GM specialist. If you want to find a specialist, here's a directory link for you: https://repairpal.com/chevrolet-repair-in-slater-iowa

The only way to really know is to scan the transmission for codes to see if a solenoid has failed, though, I have seen GM truck transmissions lose reverse as a sign that it needs some work or repair, several times.