Factory rear diff lock

I have just purchased a ford maverick swb 88 and the factory diff lock engages well but some times stays on after I turn it off and the only way to disengage it is to jack up the rear and drive it forwars and reverse for about 5 mins.

The only thing I know about it, is it works on a vacume from the intake manifold, this goes to a black box with a relay and two lies that run to the diff.

Anyone know why it gets stuck?

Thanks......