According to the salesman at Toyota of Augusta, GA the X-SP package was a way to get leather seats in a 4Runner without having to buy a Limited Edition. It is only available to Toyota SE customers.
Here is what is currently offered with the XSP Package:
X-SP w/Suede/Leather Faced Seats Pkg
MSRP: $4,010.00
* Includes: Rear Color-Keyed Spoiler
* Borla Exhaust, Carbon-Fiber Dash
* X-SP Leather Faced Seats
* X-SP Carpet and Cargo Mat,X-SP Badges
* 4-20 BFFT Wheels w/225/50-20 Tires
* (Replaces 4 factory wheels&tires)
My brother had an '06 with this package, and it looked awesome. My speculation is that they did not offer this package with the V-8 because they were trying to keep the price point below a Limited edition. Also, I don't think that Borla made an exhaust kit for the V-8 at the time this package was first introduced (someone correct me if I'm wrong). Finally, in 2006 when my brother bought his the package it came with Toyo Guard, which was an $800 option and came with a bottle of paint/finish protector that was apparently applied to the whole vehicle (this may have been dealership specific as I don't see it in the new package). At any rate it looks/sounds great and I was impressed with the dash kit.