The Detroit Locker was offered in the Spicer/Dana 60 and 70 in GM trucks well into the '70s. The "3.90" 3/4 ton axle doesn't exist as an oem option in GM products, only as an aftermarket gearset that's no longer made.
Detroit Locker units have been made for all sorts of axles, but the specific version for the Eaton banjo axles is difficult to find.
From the best information that I've found, there was a factory 3.90 gearset for the HO52/72 axle, but it's rare as hens' teeth. Supposedly, it was only available on 1970-ish big block automatic 3/4-tons. Long ago I spoke to someone who found one "in the wild," but that's the only one I've heard of. I'm sure that 1969/70/71 BBC 3/4-ton autos aren't exactly common.
