At low speeds if I hit the brakes and then step on the gas right away there is a clunk

Possible causes of this could be:

-Broken motor mount

-Broken transmissions mount

-Loose U bolts on rear axle

-Worn out/ready to fail universal joint in the drive shaft.

If it is this be aware when it does fail your drive shaft will fall out. If it's the rear joint the shaft will fall out on the road and dump fluid out of the transfer case. If it's the front joint, the front of the shaft will fall out and bounce around between the road and your truck beating it to death until you can stop. Don't wait to check this!

Rich