It's quite a distance from Horseheaven Pass to the Leatherman trailhead, if you are going to bike that and then climb Leatherman in the same day. Can be done, but you are going to be in for a long day. If my Matrix made it to the fork in the road, your Forester will be able to. At the least you should drive to Mahogany Creek, where you'll see a sign at the top of a hill saying cars and trailers are not recommended beyond that point. You just have to go slow and zig and zag around the biggest boulders. There are very few steep spots and you clearly see all the obstacles. It's kinda' more fun with a low clearance car than a truck, in a challenging sort of way (i.e., see how many times you bottom out on the way in and try to lower that number on the way out).
Lots of water along the way. If you are going to bike from Horseheaven to Leatherman, you'll first find water at Mahogany Creek. After that you will meet the Pahsimeroi before the fork.