She is probably not making it up when it comes to the speed/power limited when cold, but it would have to be ICE cold. Like 20 below zero or better, truck not plugged in, just barely starting situation. I imagine in wisconsin you get almost as cold as we do up her in northern minnesota, and these low temps might be hard for some of the southern boys on here to wrap their minds around. "Ice cold" as in 50 degree ambient outside just started the engine is a little different from -20 to -30 ambient just started the engine. My 05 6.0 does this all the time. Do you run 15w40 all winter, or change to a lighter oil? Mine especially does the no go when cold when I have the 15w-40 in it. Also, as to your vacuum leak possibility, the vacuum is very simple on this truck. It only runs 3 things. the vacuum hubs, a valve on the hot side of the coolant hose going to the heater core, and into the cab to run the mix door and vent selector on your heat system. It is easy to troubleshhot with a few basic tips. The vacuum is run not by engine vacuum, but by an electric vacuum pump and canister. There is not an infinite supply of vacuum, so leaks can cause bigger problems than they would in an engine vacuum setup. On your passenger side of the engine compartment, if you turn on the key, you will hear the vacuum pump begin to fill the vacuum charge canister. It will turn off after a while if the canister reaches level. If you don't hear the pump at all, check the pump. if it keeps running, check the canister and lines running to the vacuum control module. to check your hubs for leaks, sit in the cab,turn on the engine, turn the heat controls to vent, then turn the 4x4 switch from 2wd to 4wd high. If the hubs or a line leading to them is leaking the heater will switch to defrost for up to 40 seconds, then switch back to vent. There is also a vacuum line that runs to the valve on the coolant line that runs back to the heater core. I think this is the most likely cause of the problem with cab heat. the valve should be right next to the oil fill cap. If the vacuum hose is knocked off of the valve, or if the valve does not work properly or open all the way you may lose heat in your cab. My vacuum line gets knocked off of that valve all the time, especially when a shop is working under the hood. If there were a vacuum leak on your mix door or anywhere inside the cab, I think you would be able to hear a hissing inside the cab, especially if you stick your head down on the floor on the passenger side, with the key turned on, but engine not running. I have had the mix door in my truck "slam back and forth" . It would happen in conjunction with the vent changing to defrost and with the truck in 4x4. this was because my hubs were leaking vacuum and I would lose almost all vacuum pressure while they tried to engage.
wish I could say that!