Some noise is normal. Clunking though usually isn't. When you hit the button, the tcase motor engages the front drive more. That's why it's supposed to be turned off if driving in snow for more traction. You're hearing the front drive being engaged. There are no u-joints in 07+ front driveshaft. Only the 06 had front u-joints. When was the last time the fluids were changed? I'd start with a tcase fluid service and front and rear diff service. Good luck.