(It "Sounds Like" the Wheel Lock)
one of these past posts will clue you in:
http://www.peachparts.com/shopforum/showthread.php?t=56742&highlight=steering+lock+broken
And
http://www.peachparts.com/shopforum/showthread.php?t=281861&highlight=steering+lock+broken
And
http://www.peachparts.com/shopforum/search.php?searchid=5487602
And
http://www.peachparts.com/shopforum/search.php?searchid=5487588
1.There's a possibility that your "Wheel Lock Bolt" is going south on you.
2.there's ALSO a possibility that your ignition switch "Lock Tumbler" is headed the same direction.
3. Maybe Both at Once?
Do Not Ignore these symptoms!
If the "Tumbler" locks up on you and you are unable to turn the key to the #2
position...YOU will have the devil's own time removing the "Tumbler" to replace it.
[I'd STOP driving the Chassis,until a New "Tumbler"
(and Mayhaps a New Steering Lock) are in hand for R+R !]
I've never heard of the Steering Locking Mechanism on a Mercedes "Locking"
whilst Driving...Due to Age and Wear...BUT if it did...Whoever's Driving is Mortician Bait on Wheels.
I've "Picked On" the 1985 SD for reference,since I've No Idea Your Chassis.