I don't think the TRD springs are soft. They're shorter, stiffer springs. I really doubt they would become "worn" in any way after 15k miles. I've had TRD springs on other vehicles in the past. If the feeling is predominantly towards the rear, I think the theory of your rear shocks being the issue is much more likely. That's the thing about lowering springs of any kind. They will always show up any weakness in your shocks earlier than stock springs. Don't be surprised if in another 5K miles or so u start to experience the same symptoms on your stock springs.