Engine problem

Hey hows it goin? I have a 2001 dakota 4.7 v 8 I recently had the engine rebulit after driving soft for the first 5000 kms I was on the hwy one day crusing at 100 kms an hour I punched it to pass a car and it geared down to 2nd gear reving at about 4200 rpm the engine got very load almost sounding like vtech on a honda civic, it was back firing and boging out losing all power the truck runs perfectly fine between 500 rpm and 4500 rpm after 4500 rpm the problem starts again so I brought it in had it hooked up to a computer and had everyting from injectors to the ignition coils even all the sensors they were all working fine I cant seem to figure out the problem the mechanic said it could possibly be a timing issue do u think it could be? O by the way I got 2 o2 sensors that arnt working could that have anything to do with this problem?