My RV-6 has 360 hours and for about the last 50 hours I have experienced an increasing amount of shimmy vibration on landing (I assume it's coming from the tail wheel). Also, I've noticed it's unussually difficult to get full authority tail wheel steering on the ground. The petal goes so for and stops and it's very difficult to get full authority. At the normal cam-stop point in the tailwheel, instead of breaking past, it stops hard. I have to use differential breaks to get the tail wheel to pivot over. Has anyone ever experienced this? Appreciate any advice.