I would definitely check the manufacturers rpm specs on that engine. Very few diesels were ever designed to run that high. My old engine was a Detroit 6V53 "Screamer" ...one of the loudest, highest rpm diesel engines ever built...and it was redlined at about 3500 as I recall, and that was not an engine speed to be held for long. And if that is what it reads when cruising the highway, do check to make sure the tranny is shifting into high gear.
I would also suggest finding out what the tranny's final ratio is, then the rear end. Those along with the tire height will allow you to do all the calculations for speed vs. rpm. There are several online calculators that will do all the math just by plugging in those numbers.