SALTY SHAKERS: You’ve never seen a more wild- eyed, hyper frontman than the skinny, stage-pounding Col. J.D. Wilkes of Th’ Legendary Shack Shakers. This Nashville-based group grinds together blues, rockabilly, country and rock into one fat vaudevillian act that even a musical grinch would love. It hap pens tonight at the Mercury Lounge (217 E. Houston St., at Essex Street; [212] 260-4700) and Sunday at Southpaw (125 Fifth Ave., at St. John’s Place, Brooklyn; [718] 230-0236).