My son owns a 1999 F250 4WD 7.3 truck. Yesterday when driving it started running rough, shaking like something was loose in engine. Then later when tried to start, it just gave single. He took the batteries to a shop and had tested that they were good. Has just replaced starter and cylenoid but nothing helps so far.

 

Have you tried to turn the engine over manually? If you can ,then it could be an electrical problem. If not, and the engine is seized , well lets see what your results are before we suggest anything else.

 

Has he had any overheating?

 

have you checked for fuel if it turns over it might not be getting any fuel.crank it over and open the schrader valve on the fuel filter. you should get fuel gushing out if notcheck the lift pump