It seems like the shocks are not sized correctly to provide enough resistance/dampening which allows things to bottom out.
I assume that you have check the total travel of the shocks so that it is NOT the shocks that are bottoming out.
I know that many off road vehicles have 2, 3, or 4 shocks at each corner to take care of the high un-sprung weight and the conditions where they are used.
What shocks were on the Unimog to begin with? How do the Road King shocks compare in extended and compressed size?
Perhaps you need to beef up the suspension springs.