1991 Deville Backfire

My brothers 1991 Sedan Deville is backfire.It just started today.It usually has awsome pick up but he now has to floor it to get it up to speed.Its also backfire sometimes too when he's accellerating.Its a loud pop.And also when he accellerates the whole car vibrates like an earth quake inside the car.Does anyone know what this could be?Any suggestions are appreciated.Thanks.

Could be a bad plug or plug wire. Could be timing chain. Could be a lean mixture. All these are the suspects I'd look at first.

He took it to the shop and who ever put the plugs wires in before rested them on the exuast manifold and melted them.Amazing right?There a lot of dumbasses in the world.