727 trans problem

I am hoping you guys can help. The trans in my truck lost reverse friday morning as I was leaving on my annual Snaggin' trip for spoonbill. I baacked out of the garage, noticed it didn't engage all the way, checked the fluid and all was well. Went to Wallyworld to get fishin liscense, came out and started to back out, same thing, only a little worse. Went to gas station to fill up. Started up and tried to back up, and barely got out. Went home and pulled into shop, checked fluid, checked proper linkage alignment, all was good. Went to back out of garage, and nothing. On the way home I made sure that 1st, 2nd, and3rd all worked fine by manually shifting. I gave it hell through all 3 gears and it never slipped once, and shifted perfectly. I am thinking the servo piston or the actuating arm has gave up. Any other ideas before I proceed?