Sometimes 4WD engages, sometimes it doesnÆt. I assume the light on the dash is simply connected to the lever and is not a good indicator if everything is working or not.

IÆve noticed that sometimes when I shift into 4Hi while moving, I can feel a drag on the rear end of my truck. This morning when I was supposedly in 4WD, I ended us spinning one of my rear wheels wildly on ice while my front tires were not being driven. I then heard a loud bang as 4WD seemed to engage.

I remember reading somewhere that the transfer case uses a vacuum to stay in 4WD. Is it possible there is a leak somewhere and only when the engine revved, was there enough vacuum to engage 4WD with a subsequent bang?

What tests/diagnostics can I do to try and narrow down the problem?

Thanks!