I'll jump in here, hopefully someone with experience with one will help, first are you sure the drive belt is in good shape (only use belts from Craftsman or Husqvarna) when you say slipping a little more info would help.
Is your tractor # 917.275810 ?
Does it slip going up hill, or will it not move at all ?
When problem started did transmission make strange noise ?
I looked up your transaxle it is a peerless 206-545C and it has a history of catastrophic failure but some last a very long time if not abused, with your being 10 years old it could be shot. But I would check belt first.
