Is there an advantage to adding front "lockers" with a rear LSD?

Yes, an incredible advantage. Lockers ONLY provide extra traction IF one tire will LOSE traction. With the front (IFS) suspension on the Xterra, the limited front wheel travel increases the number of situations where one tire will lose traction.

For this reason, a front locker makes an enormous difference, by helping to keep the front end pulling.

People who have added a front locker have experienced a giant leap in their capabilities, without additional breakage over rear locker applications.

Front locker equipped rigs in general are more capable than rear locker equipped rigs, despite the old fashioned tradition of rear lockers first, based mostly upon the steering difficulties the older systems used to present on a front application.

As with ANY locker, judicious use of the go go juice is required to prevent breakage.