Evan's Creek ORV park is probably a little over your head due to tight trails and the "possibility" of body damage.
I haven't been down to Straddleline for ages.
Tahuya ORV park would probably be my suggestion. I find it really boring now but it's a good place to go to get your feet wet. Speaking of which, watch the deep puddles.
There are also trails in Eastern WA at Liberty and a few in the Manastash Ridge area, plus the Naches Pass Trail that would be fine. In fact, many moons ago I ran the Naches Pass trail in my old stock XJ on bald street tires. It handled it great. But that trail is going to close for the season in mid-Nov.
The site nwjeepn.com
has tons of great 4x4 trails listed around the state. And their difficulty ratings I think take into account stock vs. modified rigs and trail conditions. They rate things on the conservative side.
Also, I'd recommend you NOT go alone. Bad idea. Team up with another rig. Many of the guys on the board here head out pretty frequently.