You say your runiing 2,100-2,200 rpms @62 mph with the trailer.
Take the truck for a ride with out the trailer and run the same rpms and let me know what speed you are getting.

 

Without the trailer 60MPH around 1700 RPM 65MPH around 1800RMP and 70 MPH right around 1900 RPM. It shifts around 2100 to 2200 RMPs.

 

70 mph @ 1,900 rpm is about right for a 3.73 gear in od that is what mine runs.
You have a 500 rpm differance at the same speed between load and no load.
62 mph @ 2,200 rpm in the same truck and gear with a load on says that there is slippage in the trans or the TQ clutch.

Sorry for the bad news but this is my best guess with the info available.
You might want to double check with Drew to see if he has any other ideas.

I hope I am wrong for your sake, but I don't think so.

The previouse owner might have known of the trany problems and that is why they sold the truck.

 

You might check the airbox to make sure there in not 6.0 stuck in there. Maybe you sucked one up.....LOL....

 

Captain S is probably right, but the OD may not be clicking in. Try towing your trailer @62 MPH check your RPM, then push the OD lockout and see if the RPM changes. The RPM you are showing is about right for no OD.

If the OD is not kicking in, there may be a linkage or other problem, it should shift into OD well below 2,100 RPM if there is no load on the engine.

If you do need to do the Transmission, be sure to go to a Billet Torque Converter and Beefed up Transmission. The stock unit just won't stand up to hard usage and lots of towing.

 

Thanks for all the help guys. I am going to tow the trailer tomorrow moving some cattle around and should have a fairly heavy load. I will record the speed and rpms and post them here. The truck is going to the shop Monday morning so I appreciate any insight. I kind of want to know what I am probably looking at before I take it to them so I can make sure I am getting the right story and so I can help them understand my problem.

 

Check your exhaust manifold my brother-in-law had a similar problem turned out that his manifold was cracked. He was loosing boost, turns out that it is a common problem. he want with a multi piece manifold instead of stock to replace it.

 

I have heard of this issue as well. . .this way you are not getting enough drive pressure to get the boost numbers needed to make power.

also you may need to check your lift pump and see if it is pushing enough. . .that may be your problem and would definately be something to change out before you likk your VP44 injector pump.

 

I finally got some time to get it worked on and I have noticed some improved performance after changing the fuel filter/oil filter/and air filter. I had a shop look it over for anything that looked like it might be causing it but they said they could not find anything that looked wrong. I think a big part of it may have been a combination of the fuel fitler and the air filter. An after market filter had been put on and I could not identify what it was. It has no part numbers or anything on it. The shop said that it looked like the engine wasn't getting enough air through that filter and recommended finding a factory air cleaner box to put back on it.

 

Does "checking it " simply mean just looking at it for obvious cracks, or are you hearing some sort of exhaust leak from it while the truck is running?