01 s10 4 wheel drive won't engage

I have an 01 s10 with the 3 switch 4x4. When I switch into 4hi I hear the click at the dash and I hear the switch on the transfer case but it doesn't engage in 4x4. The lights o. The dash act normal and says its in 4 wheel. I've had problems with the switch on the front of the transfer case in the past and was able to clean it and it was OK for awhile and now nothing. I replaced that switch and hear it working but still not engaging. I did notice a bad vac line on the firewall but not sure where it goes yet. Is there an order I should be checking components? What should I look for next? Also should mention in 2hi I get a whining noise like a shaft is spinning but when in 4hi the noise goes away and it drives like its in 2hi...I'm confused. Its confused. Help! :runaround:

Any luck finding out what was wrong my 02 s10 is doing the same thing and can't figure it out either