You are in luck. There is a rock Trail between Rexburg and the freeway. Last time I ran it, I blew a rear shaft, ripped a tire off of the rim and took out a brake line. Its a fun one but can be hard to find.
Jeeps.org
Notice that there is a map link on the top of the page. Basically you head down the highway towards the freeway about halfway there, there are some roads that go off to your left. The is a sand hill out there that you can see. Drive around untill you can find the bottom of the hill that faces the road (North side). Strait out from there is the start of the trail. It starts with a steep climb into a V notch and you can bypass the notch to the right. The trail goes off to the right and there should be enough tracks to follow it.
I plan on running it again sometime this summer.