Since it's a late '70s one has to assume that it has the Borg Warner 13-36 Quadratrac tcase. Most likely the "failure" is just a bad chain. They do wear out and replacements are readily available. Any of the Jeep mail order catalogs or websites will have them along with just about any 4WD shop. This really is a good transfer case, although a full time case, and do well with the proper maintenance. There are kits available from Mile Marker to convert them to part time also (found at the same places as replacement chains).