Transmission shudder could be caused by several things. Very commonly it's the drive shafts, I've just had a uni replaced on my front shaft. However there's also the chance that it's an issue with the torque converter. My gearbox gave me some vibration while locking the torque converter clutch (TCC) and an automatic specialist checked it and determined that the clutch was slipping even when engaged, so the plates were quite worn which was typical for a vehicle used for towing like mine. You could get yours checked but be very wary of dealing with an auto specialist in the Maitland area.