forktrkman;
Yes, is the answer to your question, is the truck in question operating in terrain that has lots of hills and or ramps? We have dealth with this issue since 2007. Do you have a s/n. so a comparison of the trucks can be made.

How are the bolts secured?

Just torqued up
Lockwire
Threadlock compound
Tab washer
Spring washer
Nordlock washer

Nordlock washers would be the one to go for as there reusable, ive never had a bolt come loose yet when Nordlock washers are fitted.

Have you replaced the metal/rubber articulation blocks that holds the steer axle in place? With an enviroment like that you get a lot of excessive front to rear wear.

Jack the truck up and grab the steer tire and pull,if you see the entire axle moving that may be the cause,when you pull it apart,you'll see the blocks broken on the inside, make sure you order new shims for it too. I would replace the rubber snubber pad too while you have it torn down.

Also inspect the axle & mounts carefully for cracks and excessive wear.