1998 Ford F150 4X4 shift on the fly

I have a problem with the 4X4 shifting by itself from 2 HI to 4HI. I tried to see if it was the switch by unhooking it. This is not the problem. I get a clicking noise behind the tachometer when it is doing the above. I took it into the Ford garage and paid $86.00 to have them tell me that the windshield is leaking onto the wiring. It does it on dry days.