On my 91 4.0L AW4 NP242, my defrost vents blow out strong air but when I switch to the floor vents or the dash vents(the vents that blow air on your face), they will only blow out air when the engines not under load. It will automatically blow air out the defrost vents even if its not on the defrost setting. So if Im at a stop light and the floor vents setting is on, there will be air coming out on the floor where it should be, but once I give it throtle and the Jeep starts moving the air will redirect though the defrost vents. Hopefully that makes sense.
Im 99% positive I have no vacuum leaks either. Could that black canister thing that sits in the passenger side bumper be bad? I know that controls the vents.