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Looks like the only apply device that could fail and not effect the forward gears is the Low Reverse band which is located in the rear of the transmission. Going by the range reference chart you SHOULD NOT have manual first gear either. Meaning if you pull the shifter down to 1st it should not move if it is the Low Reverse band that has failed in some way. If that is the case its probably something like an anchor pin/bolt or something in that fluid circuit for the servo causing the problem.
I put an 80E in a 03 truck the first of the year because of this exact same problem. It was pushing 200k miles and the guy just wanted a GM reman with warranty and be done with it. It had the exact same problem. Just no reverse. Its got to be something in the fluid circuit then that puts fluid to the low reverse band servo when in reverse. I know there is a "reverse" check ball. Possibly an issue there? Maybe blow through the seperator plate? I hope mine doesn't do this.
You don't need the rear band for any forward gears. In manual low it only prevents overrun. So, if the band broke all forward ranges would still work identically in "D". Still could be that band, and breaking them is not uncommon especially in trucks that experience a lot of "Reverse" during a forward roll, not saying that happened.
Here's the fun part: getting to that band requires removal of literally the entire rest of the transmission to get to.