Possible there is a binding issue, something changed, with the differential brakes? Does it feel like both brakes are being applied?
To test if the nosewheel is indeed castoring, go to the outside view so you can see the wheel, apply full rudder, no brakes and aggressively apply power to get lots of air over the tail. Even without dif braking the plane should turn fairly quickly if power is applied. You should see the nose wheel trail in line with the turn.
I would test myself but not at the right computer.