Torque converter clutch (most likely). You might find it's slipping or not engaging properly.
If you are handy on the spanners, get yourself a new TC and chuck it in. Consider a low-stall-speed unit from Wholesale Automatics in Victoria, if my memory is correct it drops the stall speed from 2700rpm to 2400rpm.
Just in case though, slide underneath and give both ends of both drive shafts a good shake. There shouldn't be any movement in the unis.
Why I suspect the TCC? Because it only engages in overdrive, it disengages when the load increases so that the gearbox can kick down, and if the TCC hasn't engaged properly or is mid-engagement, it will grab&release repeatedly, which you experience as a vibration. However, drive shafts do that to, so check them first - that's free to do!
