what kind of rear differential oil should i use in a 2wd? on 2004 Dodge Dakota

occasional slight clunking noise from rear end when backing up

Asked by for the 2004 Dodge Dakota
Sounds more like the rear u-joint is failing and needs replacement, Have a shop check this before it fails and leaves you stranded. about $20 to $25 at Napa and 1/2 hour labor per u joint