You definately have a problem. You may need to take it to a good diesel mechanic and have them look it over. It's not acting like a typical Dodge. I pull an 8500 pound horse trailer when it is empty, put my 2 fat horses in there and it is well over 10k pounds. I can smoke the other people that we travel with going up hills and they are pulling the same size trailers with Ford f450's. I had very sluggish performance kinda like what you are describing until I put the Hypertech upgrade in and it was like adding several cylinders to the engine.

 

Just a thought. I had a 98 24v. that the rubber tube to the air to air got oil soaked and fell off. this truck had no power. it was a pig.

 

sounds like your lift pump is dying, get a new pump on that bad boy.