My 550 was doing the same thing. It's the clutch. What happens ( I believe ) is that the clutch sticks in the "closed" or engaged position, and will not release "open" to take the tension off of the belt. When the engine is off, the secondary is no longer engaged and if you are on reasonably flat ground you can sometimes convince it to shift, but it will eventually leave you stranded.
I highly recommend the clutch doc. Very reasonable prices and the result is a clutch that performs AS IT SHOULD.
The removal / installation takes a bit of time, and the clutch shroud is a pain to deal with, but if you have the tech manual [TM109819] things will go pretty smoothly. You'll also want to get a clutch removal bolt [JDG1641] .