Dec 29, 2008 · Jan 4, 2009. #14. The thermal actuator came first, when you engage 4WD there is a soft piece of plastic, maybe even wax that has to heat up before the actuator engages. The electric version came out in 98 or so and engages immediately. Myself, if I had that series of truck, I'd install a Posi-Lok and be done with it.