today i took my car to the shop they installed new 3.73 gears and new bearing into a mark viii alum housing. they also installed a 93 mark viii ds. driving homw was bad it was fine until about 35-40 when it would run rough so i pull over and see a bad exhaust leak coming from one of my duals about a foot from the upper cat. so would this be my problem it was also blowing smoke and would shutter at 35-40 when acelerating. the ds had new ujoints new yoke and balanced to 3500 rpms. could it be that?