I just had a similar problem on my 2005 Sea Breeze. There is a single motor with round shafts that mount to a square shaft on either side to drive the gears. The square shafts are telescoping and have set screws that hold them in place. The end of the square shaft slides over the end of the round shaft that has a square nut welded (supposedly!) to the backside of the round shaft from the motor.
If this is what happened to your rig, then all that needs to be done is to arc weld the square nut into place on the backside of the flattened drive shaft end. There is also a cotter pin that slides in the drive shaft behind the square nut.
Hope this helps.