why will my explorer only drive in 4 wheel low?

this is one i would take to my mechanic

Sounds like a problem with the selector mechanism, perhaps not engaging the 4 wheel properly. In 4-low I believe the gears are directly meshed with the transaxle which would explain why it is working in 4 Low, but not in positions that are electronically/vacuum controlled.

i had the same prob. on mine i had to change out the trans. case this costed me $600 i would check the trans. case motor first cause that only costs $180