There could be a lot of things causing this. I would try putting the rear up on jackstands and have a helper drive the truck while looking to see what is going on. Since it happened after a bump, I would check things like leaf springs, u joints, the center carrier bearing and as ou mentioned the engine and transmission mounts.
FYI, you can access the FSMs for the frontier here: https://www.nicoclub.com/nissan-service-manuals
They have sections in most sections on troubleshooting vibrations, etc.
Good luck, let us know what you find.