You can use any of the improved navigation extensions out there that allow for folded options or drop down menus.
Alternatively you can place products only in specific categories as you have mentioned. The categories will show only the products in the particular category.
Edit: To address further detail given in comments.
Create individual attributes such as truck_manufacturers, tractor_manufacturers, etc. List only the manufacturers you want in each of the attribute options. For frontend you can still give them (the attributes) all frontend label of manufacturer if you wish.
Create also attribute sets in admin and add only the specific manufacturer attribute you want for said set. Call attribute sets (tractors, trucks, etc). When creating new truck product, choose truck attribute set which has only truck_manufacturer attribute as a choice for manufacturer.
For frontend optionally use an extension that helps you display certain attributes in certain categories only. But shouldn't be necessary if products are separated by category matching (tractors, trucks, etc). Because category should only show attributes which have products that use them in that particular category.