I´m afraid that there´s rather a long reply to a short question:
If you´re using a Nikon body that can handle AF-S (Silent Wave) lenses (D1/D2/D100/D70/F5/F100) then Nikon´s new TC-14/17/20 TCs will preserve AF on AF-S lenses. No Nikon-made TC (stupidly) will preserve AF on anything other than AF-S lenses.
Third party TCs (Kenko is the brand most people mention) will drive the focusing on non-Silent Wave AF lenses.
W