We were driving down the road and stopped at a red light. When we went to start out again the truck made a loud bang like we had hit someone and wouldn't move. Some guys tried to help us push it and it was locked down tight, couldn't even push it. When the tow truck put it on the rollback he had to use skids to get it on. Don 't know where to start with this. It was running fine.

 

Rear axle. Been there, done that. My 2001 is on its fourth rear end.

 

Locked up transmission would also do that.

 

My first thought would be rear end.
Pinion, Spiders...