jrd,
well let me describe the tool (which is very simple). Like Donald mentioned before, if you can make it, all power to you.
It is a big bolt, around 8-9 inches long with two parts than screws into the bolt. One is shaped like small cup (like the tool you use to make holes for a door knob, I tried to use that too) and the other one is like a thick washer (half inch thick) which is the same diameter as the actual rear mounts. If you can find the thick washer of the same size, it may work for you. I could not find it.
As far as the actual job, you can use jack stands under the rear jack pads, a jack under the differential, loosening of the exhaust system (good time to change rubber hangers) and unbolting the THREE differential mounts (the one on top should be good). Make sure you have all the tools before you start. Once the differential is loose, you can manipulate the differential with your hands, BECAREFUL, it is still connected to the transmission, flex disks, center joint, etc. and there is very little clearance.
Good luck

__________________
J.H.
'86 300E