Happened to me as well. When i bought my Jeep it had the dent (previous owner must have backed into something). Every time I hit a bump the tailgate would make a clanking noise. After months i got tired of it and bent it back using a come-a-long hooked to the spare tire holder and a tree. I bent it back until it was straight and it dosent make noise anymore.
1998 TJ sport, 4" rough country lift, 1.25" JKS BL, Superwinch Husky 8500, 33x12.50R15 TSL radials , 4.56's, D30 Lockright