Weird Local Network Issues

At my studio we have a MacPro (running OSX 10.5.8) and two iMacs (22" running 10.5.8 and a 24" running 10.5.6). When they're all in the studio I can transfer files between each, share screens, etc., flawlessly. Everything's connected via a wireless-n router.

I had to work from home a couple days last week so I took the 24" iMac home with me. I plugged in my dinky little DSL ethernet connection, and WOW! HOLY COW! I'm still connected to work computers!!! Actually, I was connected to the 22" iMac back at the office. It shows up in the Shared section of the Finder window.

Here's the weird thing. When the Macs are all on a local network in the same building they work fine together. But when I work remotely with the 24" iMac, things only half-work. In this situation:

The 24" iMac can connect to: 22" iMac only (and not the MacPro).
The 22" iMac can connect to: MacPro only (but not the 24" iMac).
The MacPro can connect to: 22" iMac only (and not the 24" iMac).

So why is the 24" iMac able to share the 22" screen, but the 22" can't share the 24" screen? And why can't the 24" and the MacPro see each other at all? I've made sure file sharing and screen sharing is enabled for everyone on all Macs, and made sure AppleTalk was active on all Macs. Is it the difference between 10.5.6 and 10.5.8? Because I've tried Software Update, and it won't update the 24" to 10.5.8....