First off, how have I never noticed you're from DFW? Maybe you should just come up this way and help us troubleshoot!
Second, you just used a lot of abbreviations I've never seen here. PID strategy? VSS? (vehicle speed sensor?). H-bridge? (No guess on that). This might mean I'm too stupid to work on a Jeep computer....not that it will stop me.
You got the VSS. I have no idea on H bridge.
But PID stands for "proportional-integral-derivative", an extremely common control method for modulating valves. It looks at the "control variable" (on this case your rpm), and the setpoint, which is what it wants the rpm to be, and the difference between the two is the "error".
The proportional term is just a linear response to how big the error is and has a gain coefficient, output=error*gain. The problem with proportional only is it will stabilize at a steady-state error, so then we use integration of the error over a time period to nudge it to setpoint...gradually increasing or decreasing the integral output term until the error reaches zero. Derivative responds to the rate of change of the error, so if it's approaching zero pretty quickly, the derivative can go negative and cancel out part or all of the PI to gently settle on zero error instead of overshooting and oscillating.
I don't have a great mechanical analog for integral, but think of proportional like a spring (because the force applies is proportional to its compression, and the "steady state error" would be the difference between ride height and what you want ride height to be) and derivative like a damper (because it's force responds to the velocity of the piston, or the rate of change of the length).
EDIT because I didn't really finish...
From there all three terms (which each range from -100 to 100) get added together, bounded between 0 and 100% and the result is the target valve position, and it gets driven to that percentage of the total steps for that valve.
Every stepper motor I've ever worked with has to do the same thing to keep track of its position- drive toward one end of it's stroke by enough steps that it has to reach that end, and then the controller knows it's at the end. I don't know enough about the mechanical side of them but I guess overdriving it past the end doesn't damage it, it just stops. I have this type of valve in refrigeration that run 24/7 and they'll lose steps while operating and eventually get completely out of whack unless I code it to do a scheduled zero calibration, usually once a day.