NICE TRUCK!
Is it a '95?
I had your truck in a '95. It was my first Dodge Cummins. Mine was red and white. 4:10 rear. 2WD. It was a towing fool except that darn NV4500's gear spacing! Consider a gear splitter (US Gear's product suited my frame of mind) to allow you to be easier on the truck while you tow heavy. All your truck needs due to it's age is new rubber under the hood. And I mean everything, heater hoses as well and radiater hoses, serpintine fan belt, ummm, fuel hose return line, anything that comes to mind that's rubber. Then I'd change EVERYTHING that holds fluid. Coolant, engine oil (of course), NV4500 transmission oil, rear end oil, ummm, brake fluid, ummm, cup holder with fresh Gatorade! (coffee at night!), anything that holds fluid.
Then I'd tow like theres no tomorrow! Like I said, nice truck. The only REAL improvement in my '01. 5 is it's 6spd and brakes. Both of which can be improved or aided (e-brake!) on your truck!
Also run new over the road tires. You can never have too much tire!
255,000 miles? Heck, if it's had anything close to regular maintenance, it's just broke in!!!!!
Good luck!
Sincerely, Ian.