Ever lost 4WD and you didn't know why? The front driveshaft was spinning and nothing was broken...yet it didn't work regardless? Do you have an IFS Chevy or a Jeep or Dodge with Central AxleDisconnect (C.A.D.)? How'd we guess? Hmm...a correlation!
There are a lot of Chevy, Dodge, and 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 and all YJs, '85-'02 Dodges, and the '88-'98 Chevy fullsizes and all S-Series 4x4s. The Chevy fullsizes used a thermal linear actuator that engaged when it heated up (we're sure you can already figure out problems with this one), while Dodge and Jeep used a vacuum actuator that engaged when a vacuum boss in the transfer case allowed suction to come through the hoses.
Junk! All of it! All the OEMs did was add more things to go wrong with your 4WD system. But 4x4 Posi-Lok has been building solutions for the actuators for a long time now. The Posi-Lok is a replacement mechanical actuator that engages the axleshafts with a pull of a cable. The installation time on our Chevy was about two hours, and that's just because we were missing a 1/2-inch drive-deep socket. It's cake to get rid your of stock crud, and it costs less than $200 to do so.
The Posi-Lok kit is not complicated. A pull-cable actuator comes with brackets to place it on the front differential. The cable pushes a pin that presses on a shift fork to engage a collar over the outer passenger axleshaft and the intermediate shaft. This goes for the Jeeps, the Dodges, and the Chevys. Our particular kit was to fix a '94 K-Series fullsize Blazer. | 
For our fullsize Chevy kit, the front pivoting bracket (and bellcrank) ties into the two bolts that hold the front differential passenger tube onto the frame. These are not studs, so you'll have to have a box wrench on the back of these to get them off. | 
The Posi-Lok cable has a bright-red handle that attaches to a supplied bracket below your dash. The cable feeds down to a cable bracket on the differential. The cable itself attaches to the end of the bellcrank and with a push or pull engages or disengages the pushrod from upon the shift fork. |
Here is the stock thermal linear actuator (arrow) next to the new Posi-Lok pushrod, dust boot, and adapter housing. You simply unplug the thermal linear actuator, tuck the other side of the harness away in the wire loom, and unscrew it from the differential using a big box wrench or adjustable wrench. The Posi-Lok assembly fits directly in its place. Add some grease onto the pushrod and some RTV sealant to the housing threads, and you should be good to go. | 
When everything is said and done, your finished product should look like this for a Chevy. The same idea is incorporated into all of 4x4 Posi-Lok's kits. If you're still scratching your head after seeing this image, you might want to think about getting a more knowledgeable wrencher to help you complete the project. | |