Do you have an IFS Chevy or a Jeep or Dodge with C.A.D. (Central Axle Disconnect)? Ever lost 4WD and you didn't know why...and the front driveshaft was spinning and nothing was broken...yet you still did not have 4WD?
There are a lot of Chevy, Dodge & Jeep 4x4s running around with some ill-conceived 4WD systems-trucks with 4WD actuators that engage the passenger axleshaft to the differential, utilized in an effort to minimize the front drivetrain's impact on fuel economy. These actuators can be found on early XJs & all YJs, Dodges from '85-'02, and the '88-'98 Chevy fullsizes & all S-Series Chevy midsize trucks. Chevy fullsize trucks use a thermal linear actuator that engages when it heated up (we're sure you can already figure out problems with this one), while Dodge & Jeep used a vacuum actuator that engaged when a vacuum boss in the transfer case allows a suction to come through the hoses
We curse all these systems! All the OEMs did was add more things to go wrong with your 4WD system.
Posi-Lok has been building solutions for the actuators for a long time now. Their replacement unit consists of a mechanical actuator that engages the axleshafts with a pull of a cable