I'd say the best place to start is - what year is your Vitara - you're quoting prices in $ so I'm guessing you're in North America, pre 99 Vitaras don't have the air pump, 99 and later do - also for a post 99 Vitara, upto perhaps '01 there was a separate 4WD control module and after that the ECU handled it - the 4WD light will flash under certain fault conditions, but not because there was any fault detection logic - it's driven from a pressure switch in the pump, if it builds pressure the light comes on, if it doesn't the light stays off, and if it's leaky, the light blinks because the pump turns on and off.
It's a fairly simple system - so rather than throwing parts at it - troubleshoot it ...
Assuming your is '99 or later, wind the window down, switch the ignition on (don't start the engine) and shift from 2HI to 4HI, you should hear the pump run for a few seconds (no more than 10), or you can unplug the pump and connect a 12v test light between the black & red wires and that should turn on for the same 10 seconds - note - the light will not stay on, because the controller will turn it off after 10 seconds.
If the pump runs/light turns on that eliminates the switch on the transfer case and the 4WD control module as the source of the problem.
ok - you said yours is 2000 - so yours has a separate 4WD controller, not the ECU.