Kingofdoghouse

Here we go,
I have read the threads involving no reverse with retaining ring broken in OD.
I will start at the top. My son was out for a night of mudding drowned the alt. battery went dead jumped it off got to a friends house. The next day I put a hot battery on it to get home, NO REVERSE! he drove it home about 8 miles shifted fine. checked fluid level OK. put it in R nothing put it in N went forward. It only cranks in park and neutral, back up lights come on in R. Here's the funny part I went to pull him back with my truck with the thing running and reverse and it wouldn't move, I was dragging it! Shut the engine off and was able to pull him??? I haven't dropped the pan yet, thought I would ask for some insight.
Broken retaining ring?? Any help would be greatly appreciated.
THANKS
Hey Guys, I see that no one has responded to my post. Does nobody have anything to add? Dropping the pan tomorrow, see what I can see.
Am I not doing this right? First time to be on a forum of any kind. If all else fails I guess I can read the instructions, Nah!