You need the kickdown switch if for no other reason than to be able to disengage the cable while the vehicle is moving along. The cable cannot be disengaged while the vehicle is moving. You have to come to a complete stop before you pull the cable to disengage overdrive, UNLESS you engage the kickdown and pull the cable out while doing so. Then you would be in direct drive again. ...Belive me you will need to that from time to time...
I've never had to kick it down to lock out the overdrive while the truck was rolling. As long as you're under 28 mph (give or take depending on tire diameter), and the solenoid isn't engaged, you can shift the lockout wile rolling no problem. I do find it a good idea to pus the clutch in when doing it, though.
