98 4 wheel drive problem

My 98 blazer has been giving me trouble in 4 wheel drive.

first it seems to take for ever to switch to 4 wheel when moving, it will eventually, but the light will not stay on. It seems to switch right away if I am stopped.

Second, when trying to go back to 2 wheel drive it will take foreve and then when it finally does it will bang loudly when I am moving, it does seem to switch when I am stopped though.

I have the 3 button switch on the fly system in my truck. What could this be and about what would it cost to fix?

Thanks
Niels

More than likely it is either a leaking vac actuator (under the bat) leaking vac hose leading to the vac switch on top of the transfer case or the actual vac switch on the transfer case. All easily fixable.