Two equal length sockets in a vice and either a C-clamp or C-Clamp style vice-grips as the "3rd leg". Compress the lower part of the retainer in the vice using the two sockets as spacer between the vice jaw and the retainer. Compress the other side of the retainer with the clamp. Do it all slowly a little at a time. Then remove the snap ring. It's easier with an extra set of hands during reassembly but it can be done by yourself if your patient.
I rebuilt many a trans this way when i didn't have a tool.
Doug