I agree with VehiGAZ and mbeach that 4hi would be bad in dry situations, and the TOD handles theses with ease. It is the times when TOD is overactive, trying too hard on snow and ice that we are looking to correct. Not to mention the need of the mechanically inclined to have COMPLETE control over the machines they use.
So the 555 you speak of is some sort of signal generator capable of creating a PWM signal? Can it modulate it too?
The 555 is a timer chip. You can use it along with capacitance, resistance and a transistor to create a PWM signal. This signal is varied by varying a control voltage into the 555 chip. So yes it is exatly what you speak of.

I agree with mbeach's assesment of the trooper mod. The coil is not meant for the prolonged current a constant 12V source provides. And a PWM signal would be more inline with an OEM solution.

I was working toward a knob with 5 poles (and thus 5 positions): Auto, 0% torque, 15% torque, 30% torque and 50% torque. This makes wiring the lights up easier (since youre working with poles instead of an infinte knob). And it can be wired to elimate the manual button.

I really need to clean up my circuit diagram so I can post it.