In all of the designs that I have seen for rockie bogie suspensions, each wheel has a separate motor. Can a rover be driven with less motors? Does each wheel need to spin independently?
Not necessarily. But not having independent motors would cause a lot of traction from rotating the body around the axis when you use differential drive.
The dimensions of the rover matters. Having a long body might not be favourable with only 2 motors at the back. The traction can be compensated by adding weight on the side with the motors.
It is definitely recommended having a separate motor driving each wheel of your robotic system.