couple of achers has said it all. If you are still stuck, try taking pictures. Also, I noticed with my bushhog tiller, if the tiller is set on a pallet, its harder to slide the collar off due to the space restraints on the pto shield on the BX even with it swung up.
I can imagine if the auger is still attached to the gearbox , the angle of PPD shaft is making it feel like next to impossible. If all else fails, take the auger off the PD and lower it all the way to the ground and try again. I have a feeling its the combination of a sharp angle and the BX's pto sheild that in the way.