Best thing I recommend is getting a J.I. Case factory service manual for the VAC. They are on eBay frequently. The selector gear is in the steering gear box I believe so you would really need a manual before you start tearing it down. I remember my Dad saying how easy it was to reposition the selector on our SC, but then he had the manual. Did you check the front wheel bearings for slop? They are tapered Timken bearings and can be tightend some but if there is too much slop then they may need to be replaced.
That would be my first check.