You have many options here.

1. You could use the querystring approach.  Write your content module to accept a querystring value.  When you click on something in your menu it reposts the page with the value in the url.  Grab the querystring value and your set.
2. Use IMC.  If you create two view modules, you can have one send data to the other.  This won't mess with your querystring.
3. Use onclick/onselectedindexchanged events.  When you click something, something else happens.
4. Use Atlas.  Have you menu control talk to your content control.  No postback, very elegant... most difficult to deploy if you aren't familiar with Atlas.
5. Use and Atlas framework (like Telerik).  This requires a license, but Altas enabled controls are in your toolbox to handle all the Atlas plumbing.  Considering the Telerik suite also includes several DNN versioned controls, I think this is a worthy investment.

I'm sure others can suggest alternate methods as well, but any one of these will complete your task.

IMC = Inter Module Communication

This is a fairly well documented feature that goes back several versions.  IMC allows modules to talk to each other.  Traditionally, you have a sender module and a receiver module.  There's no reason why could can't go both ways, though and have one module send something to another module and that module either sends something to another module or back to the first module.  It works quite well if you are passing a small amount of info, like an id or single query item.  I don't think I would use it for sophisticated communication.  It was used quite a bit a year to two ago but dropped off the radar for awhile.  There seems to be quite a bit of chatter about it lately.  I'll post a sample later today and include the source.