The 1968 New Yorker shock that you installed does not have enough extended length for you 1971. That is your problem. Not enough suspension drop. The Gabriel's are 25.14" extended. The others are about 22-23". I hope this helps and that you have ordered the correct ones. I did plenty of...
PS. You need to check the EXTENDED shock length. The Gabriel's are about 2.5 inches longer that the Monroe's. This will give you the extra suspension drop that you probably need for east tide removal and installation.
My friend had the same problem with his Duster. Was the rear flexible brake hose changed? They clog, the pedal pressure allows fluid in, but slow to release, thus keeping the rear brakes applied. Keep us posted.
Vinny