My tractor is probably of similar age to yours... bought it new in 2002... and the linkages are basically the same. If you are like me, you use medium range a whole lot and low/high, not so much. The most obvious thing is maybe the shift linkage is bound up from dirt and lack of use. This would be really likely if the tractor is stored outside where minor corrosion can also set in at the points of wear.
Try soaking the linkages down with a generous dose of WD-40, PBBlaster, etc. If you look down in the shifter slots, you can see the linkages and where to spray. Let it soak for 10-15 minutes, then begin working the shifter back and forth over its range of movement. If it lets you shift med - high, then do that. Don't try to force it into low. Keep working it, with the engine off, in 2wd, and it will eventually loosen up enough you can try low range. You may need to gooch it with more lubricant, but it should eventually free up.
In the future, try to use the shifter a little more often, just to keep it loose, and maybe give the linkages some lubricant every once in a while.