I have had this same issue, Sierra 2500 slow speeds, especially rougher parking lots and gravel roads as well. Sounds like bumper, shocks or something loose,,,, I have a 2" dealer installed level, and was bothered by this, However, at 40k miles I replaced my Roncho "red" shocks and the noise and issue has diminished, but not gone. You have considerably less miles and would think your shocks would be the issue. I have heard of other " solutions" but nothing helped. Knowing that you have a lift or level helps narrow it down. "nothing loose" same here.
sadly, more often than not, other guys fix problems and never post repairs that worked updates.
so, If you find something that works, share it I'll continue to look, other than removing level.... <<<<< might be the problem.