I thought that must be what you were aiming for (because it certainly looks like a huge opportunity). I don't know whether changing the URL structure is going to help you though.
If I were working on it my ideal solution would be to rebuild on an integrated platform that could blend the 3 parts of the site using a common architecture. My choice would be drupal + drupal commerce. I'd build a common vocabulary (taxonomy) that was used for dsicussions, articles and stock items so that if users were having a discussion about tuning a Nissan 200sx then they'd see the top selling tuning components for that car. Likewise I'd be tying profiles back to the store (my ride) and articles back to it too.
However ideal solutions aren't cheap and you probably didn't come here looking for ways to blow 5-figure sums!
As the other end of the scale I'd be looking at what specific changes would help you hit those two goals: Better crawling of the shop. More users in the shop. There are probably some cheap easy wins there:
Home page : Spend some money on getting that right. Treat it as your website homepage not yourblog home page. Get recently added products, best sellers etc up on that page and link in to the blog less. Even if you have to manually edit it once a week that will probably be a win.
Blog: Get in the habit of ending blog posts with a related list - just link in to relevant categories of the shop. Keep the users and the link equity flowing.
Forum: Harder. You have the category block on the forum home page, but stats from forums I run suggest most people hardly use that page and don't scroll down much on it. Most come in and just hit "new posts" a lot. You could looking at some of the add-ons that let you have sponsored sections in forums and use these to run internal adverts related to each category. The tricky would be to change them frequently to stop ad blindness. Maybe mix them up with occasional big offers for members to help "train" them to look out for adverts.
I'd also look at the menu. Consider 'demoting' some items to make sure everyone knows it is shop + blog + forum. Sub menus for the shop categories would help make it easier to access the store and also help with deeper indexing.
I could go on all day. Can you tell I like the site ?