Copy/Paste this code into the Ruby Console to rename all of the model's definitions, with 3 characters off the end of their names - change i=3 to another value as desired...
m=Sketchup.active_model;m.start_operation('chop defns',true);c=0;m.definitions.each{|d|i=3;n=d.name;(1.times{n=n.chop};d.name=n;c+=1)};puts"#{c} definition-names chopped.";m.commit_operation;
Or use this code to strip 5 characters off the names' beginnings - again edit i=5 as desired...
m=Sketchup.active_model;m.start_operation('trim defns',true);c=0;m.definitions.each{|d|i=5;n=d.name;n=n[i..-1];d.name=n;c+=1)};puts"#{c} definition-names trimmed.";m.commit_operation;