4x4 wont engage

I have a 1997 Honda Passport,

The vehicle is very sound,It is an automatic with the mannual 4x4 shifter in the floor next to the regular shifter.I can put it in 4low and it will gear down really low just like it should .4l,4h,and2h all seem to shift fine with the 4x4 shifter.But the front wheels will not engage,I have checked all fuses,all relays.But it does have a Shift on the fly switch that I havent checked.The reason I think its something besides the transfer case or transmission is it shifts out perfect.The girl I purchased it from thought the transmission went out.But she had it in 4x4 low.She told me that she drove it to the store after her boyfriend got out of it.They found out he put it in 4l and wanted to buy it.Because he called me mad.and wanted it back.The girl said she drove it about fifty mph in 4l.Can that make the switch go bad.Or what could be wrong.