It has 198k miles...started running rough like a miss fire...changed plugs but no help... I've noticed I'm losing coolent about 2 gallons every 600 miles... Love this truck but I'm frustrated....any advice?

 

Time for a compression test and a hydrocarbon test on the coolant, this sounds like a blown head gasket.

 

x2 to what glc said.
My 97 4.6 blew a head gasket and never really showed much coolant loss or white smoke but the cooling system was over pressurizing and I was getting a periodic spike in the coolant temp once in awhile as well. Two gallons of coolant every 600 miles is a lot. As glc said a hydrocarbon test will tell you if you are getting hydrocarbons (combustion chamber gases) in the cooling system which is a definite indicator of a blown head gasket. If you decide to change the head gasket "just" change both side gaskets while you are at it... Good luck.