1991 6.2 GMC Suburban loss of power.
 

I've got a 91 Suburban 2500 with a Humvee 6.2 diesel in it...no EGR valve. I've put over 70,000 miles on the motor and it has always ran great. It has just recently lost power. I've changed the fuel filter and used some BG fuel injector cleaner and that didn't help. It seems fine leaving from a dead stop but once the RPMs are over roughly 2000 there's no power. Going up a hill is extremely painful. There's a mountain pass near me that I could do 45-50mph and now it only does 25. I usually get right around 20-21mpg. I just went on a 275 mile trip and went through about 35 gallons of fuel. I had to keep it floored on the interstate almost the entire time. Oil pressure was normal, right about 45psi. Temp was normal. And there was not any smoke coming out of either exhaust pipe. And no fuel leaks.
I replaced both mufflers 15,000 miles ago. It sounds like I have holes all throughout my exhaust system when I'm on the interstate but I don't.
I could understand going through a ton of fuel if I was towing a heavy load and clouds if black smoke was pouring out of the exhaust but I want towing and there was not any smoke.
Any ideas of what it could be?

It sounds like you are sucking air into the fuel supply line the to me do you have a piece of clear fuel line to you IP? This will allow you to see if this is the case.