Pet peeve of mine. I have to bring this up. When I took my factory hubs off to clean and grease them while greasing my bearings a few years ago, I read on the inside of the hubs "Warn". Our factory hubs ARE warn! They are not crap hubs! However the system that holds the vacuum TO the hubs are a different story I must admit. If there's a vacuum leak ANYWHERE in the 4wd system, both your hubs will not engage. Sometimes a small leak will create intermittent engage/disengage issues on one or both hubs. Also, the 4wd hubs' vacuum will be lost if you are excessively holding the throttle at or near wide open throttle (WOT). The WOT condition means the engine cannot create vacuum in the intake cause the throttle body is completely open. As soon as you let off the gas the hubs re engange. Most the time you don't notice a disengage though because most the time your in WOT in 4wd, the drivetrain holds the hubs in 4wd with tension. 4wd disengages best when coasting or lightly tapping the gas..
Now that doesn't answer why your 4wd would automatically engage itself even in 2wd.. Here's the likely answer, Either you have an internal hub failure or your 4wd solenoid is not closing off completely. See, when going back to 2wd, that's when the solenoid is put to work, it has to keep the vacuum from pulling air past it, but when it's in 4wd it opens and allows the air from the hubs to pass the solenoid and enter the engine.. 4wheeling is a dusty pleasure so those hubs are prone to sucking in dust which in turn passes up the vacuum lines to the solenoid and partially blocks the seal inside that is supposed to seal off the 4wd system while in 2wd. This partial vacuum can be enough to pull the hubs in and out of engagement.. this is detrimental to the hubs since the transfer case isn't spinning the front drive train. So the hubs try to engage spinning wheels onto front axles that are not spinning at all.. It can make a random squeeling noise (splines rubbing but not engaging) or a loud clunk (sudden engagement). Anyway, for my story it was a happy and I have had working OE (warn) hubs since I've bought it nearly 6.5 years ago. The problem was solved for me by replacing the cracked vacuum lines in the wheel well (cheap vacuum hose) and blowing pressurized shop air through the solenoid both ways. The solenoid is located near the brake master cylinder just in front and to the passenger side of it i think if I remember right. Sorry for the novel but I've personally experienced what i'm mentioning. The other end of the problem which I read on here to be more common is the actual hub seals seal. usually it's after someone worked on it but the driver never correlates it to the last service because they haven't used the 4wd for 4 months or more. Example.. turning the rotors on a sportage on the bench lathe means you must pull the hubs off. If you did a front brake service in may, it's possible to have a damaged seal and not know it till november when snow falls. Again, sorry for novel. <img src="/forums/images/graemlins/drunk.gif" alt="" />