Smoking white smoke out the exhaust is a blown head gasket.
It is possible the head may be cracked also. It would have to be magna fluxed to check for a crack.
If the rig has an automatic tranny, look at the vacuum modulator on the trans, it could be leaking trans fluid into the vacuum port and getting sucked into the engine. If that checks good then have a block test performed, if it is positive then you will have to pull the head and find out if it is a blown head gasket or a cracked head, both are common so you won't know until you get it torn down. If the block check is negative then look for fuel contamination, Diesel in the gas can cause white smoke but usually it doesn't run well.