Not having some of the more "modern" features isn't a bad thing. You can highlight this quality content using a more personal touch than random upvotes can provide and you have several options to do so, depending on the type of control you have over the system.
Set up an area of your message board where you and your team high light quality content. Once a week (or so), select a few posts that you think are high quality. Provide a short sentence or two on why you think this particular post is high quality and then provide a link to the entire post so that users can see the full context.
I recommend not shoehorning posts into this section if you feel nothing qualifies.
If your community has a blog, or other area for announcements, provide a shout out to content that is high quality. This is similar to the previous suggestion, but not as "scheduled".
- Linking to high quality posts from other discussions
If a post is high quality, it probably has relevance to other discussions on the forum. If such a discussion begins, provide an organic link back to the quality content. Don't spam the link all over the board, but work it in where it is relevant.
- Highlight the content on the home page
If you have the ability to make small modifications to the system, consider a way to add a flag to a post that marks it as "high quality". On the main page, display the most recent "high quality" posts and indicate the post itself is high quality via highlighting, background color or similar. This provides a small amount of automation for you.
The important part of all of this is that you promote the high quality posts. Since your users know that it is you (and your team) that is selecting this content, you don't want it to become a popularity contest. This provides an incentive to users to get recognized ("Can I post content that a team member considers high quality?"). If a week is slow, don't promote a post that wouldn't normally be considered high quality. It is ok to miss a few weeks. It shows your users that you value the great content not the amount of content.