Father in law has a pampered John Deere Trail Buck which I think is the Same as the Quest?

Runs fine but now for some reason the front end of the 4X4 doesnt engage. I dont know the whole story but it sounds like the front driveshaft is rotating as normal. Its as if the splines on the end of the driveshaft are completly worn down. (actually we dont know if this is a splined connection or not, I only mention this to explain the "jest" of the issue)

I guess it makes a grinding or a clunking noise coming from the front.

Something is up and hoping someone here knows a place to start. I think this 4X4 system has an automatic sensing device that decides when to engage the front axle. Perhaps there is biscuis (sp?)coupling here?

Thanks in advance