Watt's linkage

[100SRV]

My Bowler has an unnerving habit when under heavy braking - it starts to shimmy, only slightly but it could be easily turned into a tank slapper or Scandinavian flick. I don't think there is a problem with the caster angle as it isn't lifted (although the front sits a bit higher than a Range Rover would). This is not a new phenomenon and I am beginning to suspect that worn trailing arm bushes on the chassis bracket are to blame (although it isn't much better when the bushes are new). The "A" frame chassis bushes are fine as is the steering gear and the front suspension bushes. Is this something that I could mitigate by fitting a Watt's linkage to the back axle? Or would a panhard rod suffice?