Rear Diff. Bearings- Where should I Order from?

I've never heard of that dealer, but Courtesyparts.com and nissanpartsdeal.com and both reputable places to buy genuine Nissan parts if that's what you want to use. Both will likely be cheaper than the local dealer but you should have them give you the prices, then check those websites to make sure.

If it were me, for something like this I'd order a quality aftermarket piece from rock auto and be done with it.

What bearings specifically need replaced?

Dana 44s were not used in the first gen frontier.

Go to the thread stuck at the top f the first gen forum titled "hardbody and 1st gen axle info." That will tell you how to determine what Axles you have. And yes you will look at the axle code on the door sticker. That thread will help you decipher it.