Simplest solution for a disco axle, is to just lock the fork in the "engaged" position.

 

I'm new here so I gotta ask. what is a "disco" axle?

 

"Disco" is short for "disconnect" in this context. It was officially known as "CAD" - "Central Axle Disconnect."

Theoretically, it's supposed to increase fuel economy in 2WD modes. Practically, the economy gain wasn't enough to offset cost of manufacture, so it was dropped ~1992-1993.

The CAD system can be readily identified by a big "wart" on the back of the passenger side axle tube. It's a vacuum actuator that slides a fork back and forth with a collar in it, the coller engages/disengages auxiliary splines on the left-hand axle shaft.

Freewheeling only one wheel is part of why they gave it up - it's not going to gain enough in economy. They'd have been better off either disconnecting both axle shafts (which would needlessly complicate the system,) or decoupling at the side gears or pinion (which would still be of dubious utility.) The idea was to reduce rotating mass and inertia, reducing the amount of power needed to start the vehicle moving and keep it at speed (due to reduction of parasitic drag in running the front axle assembly in a "coast" mode.)

If the XJ was a thousand pounds lighter, it might have made more of a difference. The CAD was used in the D30 by Jeep, but also made appearances in full-size Dodge and Chevvy trucks in the D44 and D60. I do not believe either incarnation of the D50 used by Ford (IFS or Monobeam/SLA) had CAD.

Make sense now?

 

yeah, thanks for the explanation, so after all this vacuum talk, I have nothing to worry about since I have a 97