2.x mods
 

I am running at 3.6.8 if I download a 2.x mod will it work/install on 3.6.8??

Whats a 2.x mod?

You'll have 2 problems.

1. You can't download 2.x mods
2. It won't even be close to working. Hence the different sections for different vBulletin versions.

So nothing other then a 3.6.x mod would work on 3.6.8?

Sometimes a 3.5 mod will work with 3.6.8 and sometimes a 3.7 mod will work. It really depends - you should read the thread of the mod and sometimes people ask about particular versions and users answer whether it worked with that one or not. But, Mods were done very differently until they introduced the plugin/product system with 3.5, so it is almost certain that a Mod from 3.0 or below will not work with 3.5 or above.