Is it better to trade in a company truck after "x" number of miles or
run it in the ground?  My company has a diesel truck with about
100,000 miles that is starting to have periodic maintenance problems.
I've been told there is research that shows the break-even point where
the increasing maintenance costs (above "x" miles)versus the loss of
trade-in value with higher miles make it more economical to purchase a
new truck.

I don't think there is any one answer to your question. I did find an
interesting article that you may find useful.

http://www.uschamber.com/sb/business/P04/P04_7510.asp

Hope this helps.