'95 Legacy L wagon AWD MANUAL: Severe binding when backing/turning. I jacked up one back wheel and it only turns with great difficulty. Front wheels don't seem to bind. I think it's the center diff viscous coupling, but is there some way I can verify it's not the rear diff?

make sure the emergency brake is off, it's on the rear wheels. disconnect the driveshaft and spin the rear diff. rear diffs usually make lots of noise and rarely bind, so it's probably the center viscous but you are wise to make sure first.