dealer tells me my 99 expy is stuck in 4 wheel high because my tires are too worn down! they arnt to the wear bars yet!!!!! wth! then they tell me i need a new 200.00$ pcv valve housing!

 

Your dealer may be right. The 1999 Expedition 4x4 was equipped with a transfer case that eliminated the "2HI" function and instituted an "A4WD" feature. This feature is designed to allow the vehicle to operate in rear-wheel drive unless the 4wd system senses a significant difference in rotational speeds between the front wheels and rear wheels. The intent is to automatically trigger "4HI" if the rear wheels encounter a slippery surface and begin to spin. The system works well in that regard.

However, an undesirable side-effect is that the system will activate "4HI" if the front tires are significantly more worn (and therefore smaller in overall diameter) than the rear tires. This discrepency in relative tire diameter fools the system, as the tires on one axle are spinning faster than the tires on the other axle as the vehicle is being driven.

It is particularly critical with this model to rotate tires on a regular basis (5,000 miles) to ensure even wear and similar tire diameter.

You may have to replace all four tire$ to resolve this matter.