Often you can buy new parts for less money than used tools. I have purchased planetary reduction boxes and motors for my dozer from
http://www.eReplacementParts.com. They have parts for a wide variety of power tools. Chose what brand and model you want parts for and pick from the parts lists. Got a new gearbox for $8.95 and matching 12V motor for $7.50 - and no chuck to remove.
Ken