tim_cleav

I understand how the fourwheel drive works on this truck. & the front differential lock actuator (vaccum) is broken - the part that sits under the battery. I have the cable clamped into the locked posistion so my front diff is locked manually, then I select 4hi on my push button control inside. The four wheel works great.. at first (in the drive way that hasnt been ploughed in forever haha) & then once I get going down the road I feel and hear a clunk and the front diff somehow unlocks.. and my rear tires are only spinning.. the push button control still says im in four hi tho.. could anybody explain this to me?