Steve, the rear motor mount has some issue. Maybe a screw is missing and the other loose to behave this way.
Here is the motor mount (Part 40) which is still available through Lionel parts.
Here is the motor plate that is still available (Part 41).
The truck ears connects to the plate by a 90 degree turn and twist. Counterclockwise to remove and clockwise to attach.
I would remove the truck and see what part/screw is loose.
Another problem I see is with your locomotive cab on your shell. It's not on there quite right either.
This loco has been through some things.