I think of it this way.... gl4 is an oil. Gl5 is an oil with a ton of sulphur. In the 70s??? diffs were running gl4 (and didnt last too long either I hear) so in that case they mean I will meet gl4 specs for a diff.

But for a tranny of this design the sulphur makes things too slippery and synchros wont engage well. (A rebuilder even told me strait 30w oil will do well as no sulphur and same viscosity as a 80w90 gear oil) You wont damage anything, but if your going through the effort might as well opimize quality.

Regarding options. As above, Napa stalube.... or order online. Something like redline mtl. I also found a Dodge dealer on ebay liquidating synthetic gl4 for NV4500 transmissions at like $4 a quart. Or if you dont plan to keep the truck for long, just pour 30w oil and be done.