The 4x4 Posi-Lok replaces the existing vacuum shift motor with a new manually controlled shifter. When you shift in to 4WD the vacuum shift motor will slide a collar over the gap between the two shafts making them work as one. The 4x4 Posi-Lok replaces the failure prone vacuum system with a manual version. Now when you shift in to 4WD, the front driver side axle is turning, but the passenger side axle is not operating until you engage the 4x4 Posi-Lok handle. This works great with lockers. When the front axle is locked, you can disengage the passenger side in order to make turns without wheel hop and then engage the axle after the turn. Applications 1987-95 Wrangler YJ 1984-91 Cherokee XJ 1986-91 Comanche MJ Note: With Dana 30 Axle (CAD) SALE! 4x4 Posi-Lok for Wrangler YJ and Cherokee XJ PSL900Regular price: $229.95Sale price: $199.95 4x4 Posi-Lok for YJ and XJ with 3" to 6" of Lift 4x4 Posi-Lok for YJ and XJ with 3" to 6" of Lift PSL920Regular price: $239.95Sale price: $219.95 If you have questions you can reach us toll free at 1-800-699-5337.