Rev the engine at idle while looking under the hood. Bad mount and you will see the flex. Also, you could have broken the axle due to another problem. Now you have new axles and still have that problem. Clutch disc shreaded, grabbed metal and snapped your axel? Transmission had something pop? Hard to say just from you description. Need to do all the usual wheel speed, neutral, clutch in tests to see when and where you have the vibration.