No 2 wheel Drive

Hi,

I have a 2003 450 Foreman and I just completed putting on new front wheel bearings and ball joints and I went to put it on trailer to head out to cottage and I heard this Thunking noise then no drive in 2 wheel. However she will go in 4wheel. Any suggestions? I think maybe the rear drive shaft or something in the rear end. I say drive shaft because the noise was so close to the Transmission.