I need to switch my gear oil from 75w-140 to 75w-90 and Peak has some cheap synthetic gear oil. Made in USA, cheaper than Valvoline and Mobile1. The reason for the switch is my rear end chatters on turns when cold and once warmed up, it's just fine. I'm thinking a thinner gear oil could help this situation. Limited slip rear, yes I used Mopar friction modifier. 

My concern is structural integrity if I tap the housing. I think I'll just weld a bung on the cover

Just keep it as low as possible also be sure to an oil pan like bolt that is common so when you need one you can easily source it.

Personally I'd just pull the cover over tapping the housing.

Nut welded and painted. 80w 90 Peak LS and friction modifier. Test drive tomorrow.