I've got a 99 blazer 4.3. I've been driving it the last couple days in 4x4 Hi. When I switch back to 2 Hi There is a reverberant humming and light vibration through the floor. It stops as soon as I switch back to 2 Hi. I'm guessing I have a bad vacuum controlled switch on the T-case that is not fully disengaging the front axles. I've had blazers before but never anything this new and was looking for input on troubleshooting the problem, I'd appreciate any input.