thick white smoke

Motor flush won't help. You have a cylinder head or head gasket problem. Coolant enters into the combustion chamber, that's what you see at the exhaust as a white, thick smoke. The cyl. heads has to come off and inspected. Hopefully it's only the gasket, however the entire cooling system has to be checked to avoid overheating again. Zee

I believe the 4.0L OHV engines had a concern with head cracks, for around this model year.