It could be a bad firmware flash, but only if you had the firmware for a completely different printer type. Any of the Taz firmwares should still turn the motors correctly.
A stepper motor works by powering two separate electromagnetic coils with the 4 wires that enter it. Two for one coil, two for the other coil. In normal operation, both coils get sent a specific amount of current for a specific amount of time to make the motor turn. One coil powered will still turn, but will feel very week and will not hold position. Two coils powered turns the shaft as normal. But if one coil is in opposition to the other, you get that grinding shaking noise. Under normal circumstances unless the pinout has been altered, the Z motor pinout is pretty well set and harcoded on a Rambo. it is possible to alter it, but again that would be unusual.
I’d try reflashing again with a differnt method (with cura if you flashed directly, directly if you flashed with cura, etc.) and see if that helps. If not, the only time i’ve ever seen that particular issue is when the coils were in opposition due to incorrect wiring. If there is another cause, I don’t know what it is. Also check the rambo board to see if one of the stick on chip heat sinks fell off and is shoritng things that are important?