[Bryce2471]

I can almost guarantee that you are experiencing a worn out shifting shaft/plunger.

When you were geared so low all season, I assume that you usually used high gear, which means that the shaft was consistently exposed to large compressing forces that deformed it. You probably didn't notice the damage because not a lot of input torque was required to drive around with such a slow gear ratio. After all, you had low gear for the heavy stuff.
When you switched to the taller ratio, there was an increase in load that made the ratcheting start. The part I am referring to is made out of aluminum, and vex may be changing it to steel for future years.

There is a possibility that i got this wrong though. You can confirm my theory by disassembling the gearbox, but that's a pain...