I have a 1996 isuzu rodeo. When I first bought it I had problems with grinding in turns when in 4wd. That seems to have gone away, but this winter I started using the 4wd more. At first is was great, then it started having some front end vibration around 55 mph. Now it is doing it around 20 mph and it is not as easy to switch it back to 2wd when on the fly. What could be causing this vibration?