Hmmm - I actually have a similar problem in no heat unless I'm going above 25/30. So check this out -
The cause is this: The EGR cooler cracked and was replaced but an inline filtration wasn't done properly on removal allowing debris to enter the cooling system and help clog up the oil cooler which itself was most likely partially clogged with sand casting material that wasn't cleaned thoroughly during manufacturing. Since the coolant can't flow through the oil cooler then it can't properly cool the EGR cooler hence the cracking. In addition since it can't flow - it sits in the oil cooling passage ways until it super heats and vaporizes, blowing back into the degas bottle, puking all over the engine. As this happens there isn't enough coolant flow to properly produce enough heat for the cab. The fix is roughly $2600.
So check that out on yours and see if it solves it.
Just joking - only those of us lucky enough to own a Ford 6.0 Diesel motor get to have this much fun.
Ron