Hi Bruce.... no, probably not the stuffing box. The stuffing box is the 'water-tight' (not really watertight, but you know what I mean) seal where the prop-shaft exits the hull
. The end of the shaft is usually attached to the transmission
with a coupling. My guess is that the bolts that hold the shaft into the coupling have come loose, allowing the entire shaft/prop to slide back. The shaft (at least on my boat) is held tight in the coupling with two 'set bolts': bolts that have a pointed end and are run in tight against the shaft to hold it in place. As a side note, the shaft is also 'keyed' in position so it can't rotate. The 'set bolts' should then be wired so they can't come loose.
To check, climb in to where the shaft meets the transmission
.... I'll bet that the whole shaft just came loose at the coupling.
If so, repacking the stuffing box won't have any effect. The good news is that this isn't really uncommon and is an easy fix while still in the water