Have you tried removing LW rules from your firewall & then adding them again?

I guess you deleted LW's preferences folder? After deleting that folder, did all LW's settings within tools > options revert to default?

Which port is LW currently listening on?

When you updated Java, did you remove any other Java versions installed before installing the update?

AFAIK some Linksys routers aren't reliable in UPnP mode... have you tried port forwarding? The instructions for your router are here: Port Forwarding Limewire on the Linksys WRT54G
First step is to set up a static IP address: PortForward.com - Free Help Setting up Your Router or Firewall

LW's listening port, port in the 'manual port forward' box & port forwarded within router config must all be the same number.

1) No I never tried removing LimeWire from the MS's Firewall (exception) list. I only unchecked and re-checked the box beside LimeWire.

2) I believe so.

3) I'm not sure what that means, so I don't know how and what to check for.

4) Whenever I've installed a newer version of Java, I've made sure to uninstall the earlier one(s). But I haven't had to uninstall any earlier versions for months!

5) I've never tried port forwarding because I've never heard of it up until this moment.

When I've uninstall LimeWire, I've uninstalled it with a program called Revo Uninstaller (it deletes all of the leftover files, folders, registry keys, etc.). I leave for Cuba this weekend, and I won't return until the following weekend; so I won't be able to reply until I get back.

Sometimes deleting the preferences folder is the only fix needed... if you're sure that you deleted the folder properly (if all LW's settings don't revert to default on restart, then the folder hasn't been deleted properly) then try changing listening port... on the same page where you checked whether UPnP was enabled or Manual Port Forward

In LW tools > options > advanced > super really advanced > firewall. Change LW's listening port to 65000 (or something in the range of 49152 - 65535) > ok.

Also log into your router & double check that UPnP is still enabled within your router's configuration.

If that doesn't help, try port forwarding. Since you can't connect with other p2p apps, maybe the problem lies with your router.