I've heard that slow cookers, because they use water as a temperature conductor, don't generate enough heat for many cooking applications, including browning, sweating, whatever. The garlic may taste raw because it didn't have the time or temp to cook the rawness out of it. This would be the same reason garlic, onions etc are often sweated or browned in a pan before liquid ingredients are added.
So, can it be fixed? Maybe, maybe not. You might find a hard boil will temper some of the rawness, as the garlic may get a bit more cooked (I suspect a cooker would not rise to a hard boil to prevent burning, since slow cookers aren't stirred). The amount tempering you get this way may or may not be enough for your tastes. You might try pressure cooking, though I don't know if that will make a big difference as opposed to stovetop boiling.
As far as I know, no method of heating the soup as a whole will get you much above what boiling water can already do, at least until your soup is no longer soupish.
If the garlic was in larger pieces, you could take them out, saute, and re-add them... though I suspect if the garlic was in such large pieces you'd have already tried taking it out. Maybe if you strain all the solids and saute them? you'd get more browning in the soup, and more breakdown into little skittery bits, cloudy and mushy and stuff, and probably more oil, all of which I don't mind but you might.
If you can't get this batch to mellow enough, then you can either dilute as SgtStens mentions, or discard and start over, or find someone who adores the bite of raw garlic in their soups.