The road up to the top is fine for an unlocked stocker. No obstacles, just mud if it has rained recently.
I don't know of any extended stretches of rocks / boulders anywhere at Aetna, but if you'll continue straight on the main trail, until you get to the Lookout Valley power line overlook, turn right and drive along the powerline. You'll find a couple of stretches with rocky hillclimbs, with some ledges that are challenging for any rig with tires less than 40".
Stick to the power lines if you want rocks. The wooded trails are mostly flat and have varying degrees of muck.
Something we didn't explore much, but was getting pretty rocky when we turned around for fuel reasons, so you may want to check it out: From the Lookout Valley overlook, head northwest on the same power line I mention above. Continue after you see an intersecting power line. Eventually the driveable trail pretty much ends because of a gorge. Take the very last trail on the right and it becomes pretty rocky.
If you follow that to the north, you should end up at peanut butter hole, where you'll find rock ledges and a very cool overlook, above the Tennessee River.
I higly recommend you carry a printout of the Google map. Do a printout from terrain mode and another from satellite mode. The criss-crossing power lines can throw off your bearings. The satellite map will help you see where trails go. The terrain map will tell you if it's a freakin cliff.
