how much would it to replace a rear transmission mount in my 1994 blazer s-10

you need a mount and seal replaced I would say $300 for parts and labor

Assuming it is a 4x4, some were not, the mount is about 20.00, the seal for a New Process transfer case is about 9.00! However Borge Warner transfer case seal is about 26.00! Parts + labor no more than150.00. Not a mojor job if handy with a wrench and in this case a hammer!