The 5R55N's were known for some problems. Issues with the valve body,,, and broken springs. If you find a reputable transmission shop,,, and the trans doesn't need a full rebuild... I would at least have a full flush done... plus a new valve body, new solenoid pack, front pump seal, rear tailshaft seal. Torque converter wouldn't hurt either.
In all actuality... the whole problem could be the torque converter. Laugh if you want,,, call BS if you want. I've seen it happen. Old buddy of mine (decades ago), did a reverse drop in a 72 charger. I was in the car and heard a strange metallic "bang", (up by the engine), when it happened.
He lost reverse at that point,,, and thought the trans was fubar. I told him it sounded like the torque converter, (more than once), but he wouldn't listen. He went to the boneyard and got another trans... and put it in using the old torque converter, because he didn't want to take the time to unbolt it... and because he didn't believe me.
Guess what??? He had all the forward gears,,, but still didn't have reverse. Next time I talked to him it was fixed... after trying a second boneyard trans without luck,,, until he went back and got the torque converter from the same vehicle he got the trans from.