<b>Jake:</b> I will start this off by saying that Pennzoil has a gear oil named Gear Plus 80W90. It is a GL-5 gear oil and it has all of the posi trac additive you need. You can drain and do a refill with this product.
I'll add my preference, That is Chevron Delo Gear. The Borate/Group II combination gives more portection without harming the ssoft mettals, and work's excellently in all types of Differentials. You can see some of what the additive does on my site under "aplicaciones", or read specs on Chevron.com