I know of at least 2 different 2166's that had that problem, although both of them  finished a harvest with a strap holding them in 2nd gear (one was a local guy's and one was on an auction and the owner told me about it).  I don't know what the exact trouble was, but they had to pull the transmission and do some work in both cases.  They both had less than 2500 engine hours on them when it happened.  I can't help you with the fix, but don't let them tell you that yours is the first one that they've ever heard of it happening to.

HTH,

Mike