Not exactly yakuza, and not a tv drama, either, but the Stray Dogz (a.k.a. Yamikin Dogs) movie series is pretty neat. It stars Yamada Yuki as a young ex-yakuza boss who quits yakuza to become a loan shark, and Aoki Tsunenori as his subordinate. There are 7 films out at the moment (+parts 8&9 in the making). The first one if actually not that good but it gets much better starting from part 2.