You have Active-Trac, and it's a front axle disconnect set-up. As far as I know, you must have vacuum to the front axle disconnect to keep the system out of 4 wheel drive. So, make sure you aren't losing or leaking vacuum to the vacuum solenoid or the plumbing that goes from the motor down to the front axle. You may just have a worn vacuum motor or solenoid that lets loose only intermittantly.
Can't say just how the DOHC 3.5 engine is set-up to provide your system with vacuum, as I have the smaller 3.0 liter engine. Ask around on this board for further help.
Could also just be a linkage adjustment or switch on the top of the transferbox like you were also thinking.
Hope this helps you out.