curious, grabbed the MX6000 front axle shaft drawing and can see 2 possible reasons.
1)Looking at it ,my gut says maybe the balls have pooped out, 'somehow' ? I'm assuming this is the 'sliding' gear that transmits power to the front axle,if so,if the gear went past the 2 balls and spring, spring pushes balls out and 'locks' the gear into 4wd. Hard to believe gear could move that far though..
2) The other possibility is that the fork fingers 'magically' came out of the groove on the rear of the sliding gear #060. Say the rear lip broke off, gear could be pushed forward but NOT pulled back.
Now I've never had one apart, don't have xray eyes but it could explain the 'locked in 4wd'
If it was possible, inserting a borescope camera would be real nice. Get EYES inside BEFORE tearing the tractor down into 100s of pieces and costing $1,000s to fix.
I'm thinking #2 is the front runner
it will be interesting to hear what the dealer finds....