ray, you have to determine whether your lift is a proportional drive or a "bang-bang" drive (the SJMs were all "bang-bang" drive as I remember) and match the joystick to the drive system. If you use the wrong joystick you will go into full speed immediately.

Recheck the wires that are changed ("just a few wiring changes") where they attach, between the new style and old. I think you will find there is a wire or 2 in the wrong spots...
I would also note that there are 2 different versions of the joysticks for these units, (which one was the old joystick) one has "varible speed [potentiometer controlled lift and traction]" and the other was 2 or 3 speeds [depending on how you count these things] so it may be that you got the wrong joystick.