Found this on the rear differential and not sure just what it is for or what it does. Nothing in the Haynes book.
It sits just behind the spare tire and a hard line goes over to a bracket on the left side of the differential. As can be seen their are a few vacuum lines attached. A second line goes to the diff, right side.....
It distributes braking bias based on load (gauged by ride height) between the front and rear brakes. Some (including myself) have chosen to remove them and replace them with a manual proportioning valve.