Todd,
There is a detent position on the shifter rod (4 all together) that keeps in proper position of the shifter. The shifter locks in the position with the action of a ball and a spring that locks the shifter position. Same thing happened to me on my 1700. just below the shifter you'll see two bolts as the below picture shows, Just loosen them up, don't have to take them out all the way out( if you curious like I am,, you take the bolt off , there comes out a spring and a steel ball in the hole, just clean it up and put back together).try to shift and try all four ranges , screw the bolt in with hand pressure till you are able to shift to all four ranges and the shifter stays steady in place. I then tighten the bolts normally and start using my tractor. Hope it works for you.
JC<
P.s I just read Soundguy's comment carefully. You know he is always on right track, no difference this time either.