Error! The belt pulley gear train runs in the hyd oil, completely seperate from PTO/Tran/diff.
Hint- easiest way to get those square headed filler plugs out is to use a 12 point socket that is 1/8 in larger than the square. Eg: 3/4 sq use 7/8 12point. 1/2 sq use 5/8 12point. Then with an extension you can get ratchet above frame rail & be able to turn it easily. Beats the heck out of skinning your knuckles down inside the frame rail.
Probable the least messy to drain would be start with the trans drain. Some oil will remain in diff & PTO sections. Then go to PTO, remove 2 bolts at one end of plate, then just loosen, do not remove, the other 2 bolts, let the plate droop & guide oil into catch bucket.