I have a 99 6.0 2500 automatic plow truck. I only use it for plowing on my property in Colorado and rarely drive it above 20 mph. I was plowing today and noticed my power was greatly reduced. In gear, I could barely get it to 1500 rpm going uphill. My foot was on the floor. At the top of the hill, it picked up a little speed. In neutral, it will rev normally. No codes. Any thoughts to get my power back?

 

check fuel pressure. ever change the fuel filter?