My vehicle is a 99 jimmy with 4 button system.

Ok here is what happens, the truck starts out in 2hi I press the 4hi button,
I hear a click from the computer on the right side, now the 4hi button
continues to blink, it will continue to blink never switching back to 2hi,
now if I press the 4lo button the 4hi will stay on and the 4lo will come on,
but when I put it in gear only the rear end is in 4lo, the back tires will
spin in 4 lo but the front end has no power.

It is as if the valve in the front under the battery box isnt working or I
have a vaccum leak. Any suggestions?