Hi Matt, experiencing the same exact phenomenon with my kids Funtek MT4 4x4 RC Truck. It uses the exact same ESC/Receiver called HBX 12891 1/12 ESC Speed Controller Receiver Unit 12522RT.
It's been a while it wasn't used but was working fine before. Recharged all batteries (for the remote and the truck) but what ever set of batteries I use I have the same problem. Strange thing is does receive a signal as it stutter as you, but direction does not work at all.
Curious to know if you had any luck by replacing the motor or the radio?
I don't believe it's the motor, in my case replacing the motor wouldn't not resolve the direction problem.
Check all cables, batteries... everything is fine.
Thanks,
Jerome