HANGFIRE, there is some protection here for you provided that both forums are logging IP addresses of their registered users.
You may already know this since you've found what looks to be the registry information for the forum web site as well as it's IP address but in your defense you should use it again to prove your identity and disprove the identity of the imposter.
Whoever the imposter is at the other site will have one specific IP address associated with them each time they post, no matter if they used your email to register for those forums or not the IP address will be exclusively theirs even though the email they are using is yours.
You've been posting here for quite some time and you've likely got a decent backlog of IP addresses logged here that show where you regularly make your postings or access the internet from.
Perhaps if you're real nice to the administrators of both sites they will provide you with the evidence you need to show the authorities that you are being slandered, harrassed, molested, impersonated, or whatever you want to call it and that whatever the person says on the other forum it is not you who's doing it. If you're trying to go for proof this is likely the way you're gonna want to go about it. It does not serve the interests of either forum to have this sort of activity going on, it could create headaches down the road so releasing the IP information to you could help to solve these issues before they can become future problems.
If you've contacted the authorities and they've taken it seriously and are looking into the matter then they are already probably in the process of doing these things.
With luck the IP addresses will clearly show that while you post from one provider in one state the imposter is off in some completely other state plugging away and acting like an ass which will likely come back to haunt them at some point.
If they are really harassing you and law enforcement isn't interested in helping you then you very likely can have somebody do an identity trace on the internet provider so that you can directly contact the company and tell them one of their customers is acting in a neglegent manner, typically an internet company will up and pull the plug on the idiot and refuse service to them from that point foward. They don't want to have any hassles with people who like to use the internet to cause trouble.
Now you know why people don't like to broadcast their email out over the internet, why they take steps to remain annonmous, or why they cool their jets and not try to rub people the wrong way.