1996 Chevy S-10 4 wheel drive

1996 s10 pickup with 4.3 and auto tranny--I also have the manual floor shifter for the four wheel drive system. Most of the time when I shift from 4hi back to two wheel drive, it wont disengage the four wheel drive. Sometimes it will and when I get to highway speeds and start to slow down it acts like it has engaged again, and the hubs seem to be howling.