Compression check can be done on that engine in less than 20mins. Thats how I found the head gasket leak on my 91 4.0. I was losing coolant; not as much as you though. Rear cylinder was 30 lbs lower than the other 5. It ran perfectly too, no smoke.
You could also run it by a shop and just have them hold an exhaust gas tester over the reserve bottle or radiator with their caps off while engine is running. This could show compression blowing into the cooling system if the reading is high.
With the engine hot, pull the plugs 1 by 1. Put a 1 foot length of rubber hose (2 feet if your not very flexible yourself) tight against the spark plug hole and smell for antifreeze odor. If so it will indicate a head gasket. If you do the head gasket, have the head resurfaced just to make sure. Good luck.