Anything that is either semi or fully synthetic, from a reputable manufacturer. This is going to nearly double the price of your oil, but since we're dealing with less than 2 quarts and semi-fully synthetic oil lasts much longer and gives better protection while you're at it, it is truly value for your money.
Are you having any rear diff problems? Do you hear any knocking sound coming from the back, especially when you do a medium or soft brake ? If it is within the rear diff itself, this might be a good time to change the diff, or possibly use a thicker oil to see if the thud is mitigated.