Hi Guys,
Thank you for all the responses! Very interesting insights.
Turns out this is the 4.0 L engine and not the 3.0L
The head / headgasket is leaking. The coolant is milky, and there is bubbles in the radiator. The PO filled it with the "miracle in a bottle" stuff, so there is sand and flakes in the coolant. Also, the radiator drain does not release coolant when opened, so who knows what condition the radiator itself is in.
Currently taking off the heads, and am hoping there is no visible cracking. Debating whether I should resurface the heads (cast iron) or just clean them up and replace the gasket. My understanding is if they aren't cracked, cast iron tends to warp less than aluminum.
BTW, the spark plugs are Motorcraft Platinum SP486