If it's not an option with the other transmissions, make sure the MOD is at the top of your load order. If that doesn't work, try unsubscribing and resubscribing. If your using a Modded/added truck, it may not work. Those are the only things I can think of.

I can't seem to get it to work on any of my trucks, modded or just the factory. Motor or transmission isn't in the options. Do you need to be a certain lvl?

Not showing up on mod manger.