Try rolling the car an inch or two foward and backward
If the the tranny was low on fluid it sounds like you have a dry gear and its binding up, if not strip a spline.
But if you can wiggle it out of gear and fill her up she might be okay, but I'm not the tranny authority on this board.
Even if its a bad gear depending on where you are located a junkyard tranny has a good chance of giving you more mileage on this beast. I know that I can go to a you pull a part yard and pull a tranny for $50. and these are eaasy to replace.
But have someone in the car while someone pushes the car foward and backward to see if you can postion the gear to release .
Good luck