Locking pin or lack of doesn't cause shimmy but can cause poor ground handling and greater risk or groundloops. The geometry of the tailwheel can be a huge factor though, and I'd advise checking that. I've had two aircraft that suffered shimmy. Both were cured by "straightening" up the angle of the pivot pin by altering the tailspring angle.
SS
Edited to add ... What sort of tailwheel is it? I had a share in a Falconar with a Prestige tailwheel. The locking pin on that disintergrated and ended up fully castoring too (didn't shimmy mind).
Last edited by shortstripper; 5th Jun 2013 at 21:17.