My 4wd does not engage anymore. I have replaced the carburator, 4wd vavuum switch, 4wd actuator, and checked all of the vacuum lines (which are all good), I checked the check valves (which are all working properly). The transmission is responding like it should.I am at my wit's end. I spend more time under my jeep than I do in it. Do you have any idea why my 4wd is not engaging?