I have a bad leak from transmission on 1987 subaru gl .Could it be rear shift selenoid

It could be.....although I don't know excactly what you are refering to as a "shift solenoid".  Kickdown?  Modulator? Governor?

 

Or it could be many other things. 

 

You aren't giving us enough information to be helpful. 

 

Pics?  Description of where the leak is coming from?  Any recent work done or odd drivng?

The mechanic that works on this car called it a rear shift solenoid. Its at the very tail end of the transmission before it connects to the drive shaft of rear wheels.He cannot find one.So im looking on internet myself.