Hi. I've lived in Whatcom County for about two and a half years. Unfortunately, there aren't any public offroad areas here, but there is Walker Valley in Skagit County. It's about 45 minutes from Bellingham. There are some really fun trails out there. The last time I was there, it was raining pretty hard and my buddy and I had a blast.
The only offroad club that I know of in the area is called the Ranier Ridge Rams. Edit: I just went to their website and it looks like you already posted there.