You can find SAE 50 if you look hard for it.
I have run 50w for the summer and 30 for the winter as per the manual for years
and never had a problem with any of my EBs t/cases.
As for o/d trannies... what can you afford to do??
I currently run NV3550s in two of my EBs. I would do another in a heartbeat.
There are other cheaper options, and others that may seem cheaper, until you factor in body lifts, and driveline mods... it will also depend on what you need your truck to do.
Surf all the EB sites you can find, including vendors such as Advance Adaptors, JBG Duffs BC Broncos etc... there you will find multitudes of options.
Hmmmm, just noticed the 6 cyl thing... you may be limited a little in your choices due to the shorter tranny adaptor length of the 6 cyl....