Timber Ridge in Castana, IA is a great place to run, but it is a 'Members Only' facility.
Great trails and campsites can be purchased.
Haven't been up there in a couple years.
Central Iowa 4 Wheelers have an annual fundraising event there (which I missed again).
I usually try to find some good class B roads as well. I've ridden all of the B's in Linn county, unfortunately they seem to be closing more and more of them. 1 very small area that's still a bit of fun in the mud is next to the Cedar River in Cedar Rapids, just south of C St. on the West side of the river. It's by the roller damn and an area no bigger than 2 football fields, but its great for testing out a new setup and stuff like that. Plus its right across the road from the CR Police rifle range, so you get plenty of looks. Just make sure your rig is legal!
600 acres is the one north of IC. If you go west out of Solon on the main drag you should find it on the right. I used to take my Jeeps out there in High School. If you get stuck don't leave your rig unattended it will get stripped by them time you get back.
Lots of class B roads around North Liberty, Solon, Oxford and IC.
600 acres has been closed for the last 4+ years to jeeps, Trucks etc. The local clubs didn't want to take care of iy, MIOBI, So the ATV club said they would and ATV only it was.
There is no places in Iowa to legally wheel except small farms clubs have or private land. The only half way decent one is lake rathburn. IIRC it has light 1/2 rate trails to drive. More scenic than anything. Which there was some but SOL. Hannibal rocks, Cliffs insane terrain and badlands in Attica are your best bets. Good luck