Exactly what Mike said... with a little bit of downward momentum on the rear of the truck the frame (via bumpstops) hits the axle and it is felt and heard as a thump.

A lift will remove this problem.

This is my only complaint but I have been happy with the solution.

DO IT! You will not regret purchasing an X.

Feel free to ask any other questions, but please do a quick search on the site as it is probably covered extensively.

Good luck!