Hi,
the steering is very stiff because in 4 places Lego used friction pins instead of friction-less ones. In the 7th post of http://www.eurobricks.com/forum/index.php?showtopic=73311 you can see which parts need to be replaced. The replacement is easy, and the steering afterwards as well. Downside: when one of the front wheels hits an obstacle the steering might change.
To make it even nicer you can repalce the HOG by a servo, see http://www.eurobricks.com/forum/index.php?showtopic=57543&st=1325 and the following posts.
Have fun, the Unimog is really great.
