My '99 Diesel did this as well. The blowby tube that comes down on the front of the engine allows oil mist to get on the radiator and over the years there is a surprisingly large amount of dust and dirt that gets collected and plugs up the fins of the radiator. Way more than you would believe if you are looking at it without taking it out of the truck.
The best advise I can give you is to bite the bullet and take the radiator out of the truck and give it a very thorough cleaning. I did this and it acutally only took me about 2 hours total until it was back in the truck and getting the coolant refilled.