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You might want to try also posting this in the superduty forum too. I read up on some problems in the F150s related to a poor relay design -- maybe it's also in the SD trucks...
Do you have heat going only to defrost no matter your heat selector? If you have only defrost, then you have a leak either at the ESOF solenoid or in a vacuum line or at the wheel hub. You can check by sucking on the hose that goes from the vacuum soleinoid to the wheels, solenoid is on the Right fender, if air does not hold, you have a leak, If it holds air, you probably have a leak at the solenoid or solenoid does not work.
If you have a leak at the lines to wheel, go under the truck, the lines goes to both side, suck on one at the time and you will find which one is leaking.
If one wheel is leaking, you will have to take the hub off and change the O ring.