SANDOWN, N.H. — A New Hampshire truck driver in need of a kidney has taken his search on the road, with an ad on the back of his 18-wheeler. Mark Boomhower, of Sandown, figured people stuck in traffic would see it. The ad reads, “Kidney needed for Mark” on the back of the truck. “Would you share your spare?” The 54-year-old Boomhower tells NBC Boston he was diagnosed with kidney disease 15 years ago and is on a donor list, but it could take a while. He says his kidney function is down to 12 percent, and dialysis is looming. Boomhower is being treated at Brigham and Women’s Hospital in Boston, which runs a living kidney donation program.