Same thing here with my ~5 year old Craftsman 18Volt 1/2" Drill / Driver. Light duty occasional use, but when I went to use it, the gears were stripped. No repair parts, and no response to repair requests sent to Sears. No outside shop will work on it as they too can not get parts.
I bought a DeWalt 18Volt 1/2" as a replacement. On-sale at ~$100.00, and another $50 off if I applied for one of their credit cards.
No brainer to me, but everyone's mileage will probably vary!
p.s.: What do I do with the Craftsman carcass & batteries?