What would cause my 4.0 to white smoke, and have a sputter sometimes and gets horrible MPG but not all the time?

 

Sounds like a head gasket, are you losing coolant? And heck the oil. If its milky dont drive it!

 

Smell the smoke, put your hand on the way of the exhaust and then smell your hand.
If it smells more sweet then you have antifreeze in there somewhere.

Head gasket as mentioned above.

 

Its not loosing coolant and oil is good

 

you may not be losing coolant with a bad head gasket but the cooling system may be building pressure from compression leaking into the coolant passages. i would put a pressure tester on the rad and run the truck for a while ad see what kind of psi readings you are getting

 

I think the truck is over fueling, it smells like gas and it nly smokes after it has been running for a few mins and when you push the gas, could one of the injectors sticking cause this?