Apparently this is a common problem with this machine but surprisingly I couldn't find a whole lot of DIY help online so I went through the steps I took in order to diagnose the problem. The tech manual for this machine is very helpful on this topic and if you do find that it's your 4wd actuator that is the problem (like I did) then this video may be of use to you of you've never had to remove and install an actuator before.