Whoever fitted the propshaft got it wrong. As Lew says you need a proptube with bearings to carry the propshaft which should be long enough to allow for the prop, locknut and thrust washer at one end and a thrust washer, lock nut and connector at the other end. There should be a small amount of play between the thrust washers and the proptube bearings.
This set up takes all the forces from the prop in either direction and prevents any force on the coupling or motor.
If you don't want to reduce the propshaft length then a lockable collar and thrust washer at the motor end as Nick says will also work. You also need to fully engage the splines in the U/j fitting, moving the motor and realigning if necessary.