737 operators have drills for dealing with a 'uncommnaded yaw or roll' or 'uncommanded rudder'. This involves using all available flight controls (ailerons and rudders) to maintain control of the aircraft. If necessary we will lower the nose to pick up airspeed (hence making the controls more effective).
Most operators also have an 'advanced manouevre program.' This involves a whole sim session of recovering from 'unusual attitudes'. Great fun! - and very useful too.
Part of the problem with one of the 'uncommanded yaw/roll' accidents (possible due to wake turbulence) was innapropriate recovery (not rolling wings level befpre attempting to pull out of the steep dive). This is something we are practised at.
There is certainly evidence of rudder hard overs in the past, but Boeing strongly deny much of the speculation and say it is perfectly safe.