Actually overheating under load is probably a radiator.
Put in a Nissens from Rockauto.com - about $250 for a new radiator.

Also under load when getting hot your fan clutch should be really roaring.

 

I have both top and bottom fan shroud on. The fan clutch roars and stiff at cold and hot. The dex-cool is less than a month old and new to my system. Before was green. My mistake. The radiator is Nissen brand. Water pump is fairly new as well. I'm leaning towards the only old system on the truck is the Y-pipe back and muffler.

 

Just to pile onto this thread. I'm experiencing a similar problem. No coolant loss, new radiator, hoses, new fan clutch, new 180T genuine tstat, new expansion tank and cap. Temperatures continue to rise under load unless I take action.

Last night I was driving up a very long but not too steep grade on I90 up to Snoqualmie Pass. No AC on and temps continued to climb all the way up to 221, at which point I pulled over to the shoulder to let the truck cool, before continuing on my way.

I was only doing 50mph at the point where my cooling system couldn't keep up. The fan was running the whole time and temperatures quickly started to drop once i pulled over and let the truck idle for a few minutes.

I have an 04 4.6 (no oil cooler). over sized tires, stock gearing and a lot of heavy metal bolted onto my truck.

Theories?

 

What was the outside air temp when you drove up the pass?

As I mentioned in the other thread on this subject I recently noticed higher than normal temps while air temps were above 92 and I was driving > 70mph or at a constant 2500rpm or better... Turning the AC on and off changed the temp by 4 to 6 degrees. Everything normal in city driving even in high heat though again above 92 the coolant temp went up to 199 instead of 197 at idle in traffic. Turning off the AC in city traffic got the temps back down in the low 190's in high heat city driving.

Interesting to note that there apparently is a notable horsepower price for keeping cool.

 

Come to find out that my right side cat was bad. Cause of overheating under load. It cools itself down when driving or idle. Replaced my y-pipe, muffler and rear pipe and runs cooler than before. But look at my liking. I later found that my clutch fan was keeping up in 100 plus weather with no ac. Spikes uncontrollable with ac on. Electric fans works hard. I replaced my stock single piece fan clutch with a #ERR3443 and 11 blade fan I had. It's cooler by a couple of degrees. But I believe a severe will do the trick. How will you put these clutches in order?

1. Hayden #2991 - Online
2. Imperial #215158 - somewhere
3. Thermostatic #271312 - Napa
4. #ERR3443 - Online
5. TorqFlo #922744(Heavy) or #922786(severe) - Autozone