I have a cforce 500 that works great except sometimes it won't disengage 4x4. Normally I can hear the switch from 4x2 to 4x4 but when I hit the button nothing happens. The last time this happened after about a hour of use it resolved itself but I'd like to know the reason behind it. Currently, it's stuck in 4x4 and I can't disengage it or lock the diffs. I don't hear the motor trying to even switch.

I haven't had to deal with this problem yet. I would have to think that its a electrical problem. Download the manual and start to check the circuit try the one NL800 posted, that is for the EFI model.

Did you ever get this fixed?

you need to move it before it can unlock from 4x4
else the front sensor/gear may have gone faulty.
you can have it check to see if the sensor is faulty.

Did you have to move it before. Some machines need it start moving before 4wd will disengage.