Hi Joe, and welcome to MLMF!
I tend to agree with Mark / Ohio about 'chaff' plugging the radiator cooling fins. These models have the incredible Yanmar three cylinder, diesel engine and are tenacious, durable and reliable. But keeping them cool under heavy loads is a must. Even the weight of the engine oil can raise the temperature several degrees - Check this out:
TractorData.com John Deere 455 tractor engine information
What you have, is not a typical LT that has a life expectancy of 5-6 years before contemplating the overhaul on other models with gasoline engines...but a diesel that will go decades
before the top end might need rebuilding. This is, of course, based on a cool running Yanmar diesel. If it were mine, I would check all the hoses, belt tension, thermostat (if so equipped) and air way passages to make sure something isn't causing the problems.
Good luck and let us know what you find.