My Taco-driving buddy lives in Seattle and his 2006 had the "bad" rear springs and I recall they were subject to that TSB. He had a good and longstanding business relationship with his dealership, right down to them performing all scheduled maintenance. The outcome for him was, I believe, no charge for the replacement rear springs and all related mounting pieces and parts, and something like $600 labor charge which he paid.
He's now semi-retired and keeps enough gear (plus the topper's weight) in his Taco to max out the GVWR, so he's very happy with his upgraded rear springs and his E-rated Michelin rubber.
Foy