Mine did this and like everyone says check puke bottle. Mine got full and covered rad with oil then it gets dirt on oil and stops rad up solid. I pulled my rad and pressure washed the fins out, it dropped 20-25 degrees off temp gauge pulling my 36 foot toy hauler.

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Check the square vacuum line block on top of transfer case. Mine had got all soft from oil though the years and was losing vacuum. I replaced it and got my 4x4 back.