Hi maybe i can throw some light on the problem, in as much that my pto would engage in the 1000 but not the 540, i decided to look inside the PTO & after checking all the linkages, it meant a complete strip down of the rear end to gain access, what i found inside would make you cry, the main gearing was completely stripped, the problem was caused by the inner shift arm , this keeps it in one of the three positions by a ball bearing under preasure by a spring, which slides up and down the selector rod, this had got stuck by some debri from the gears keeping the pto from selecting the 540, the problem with yours will be the opposite, in that the selector rod within has locked in to the shaft thus keeping it in drive, after the initial strip down getting at the pto was a walk in the park,. i hope this may help in a small way ..