SunDude, they are 28's.
The Driveshaft Shop
http://www.driveshaftshop.com/ sells 31 spline half shafts, but be careful because I've heard a few situations where the replacements were not EXACTLY what was needed. Some SPF owners have combined the 31's with a Detroit Tru-Trac (sp?) posi member and this combination is about as bullet-proof as you can make the 8.8.
Also add a Steeda brace to the rear cover (see my gallery for pics). Gear deflection under HARD acceleration has been known to crack the rear cover in extreme instances. I added the brace for insurance.
-Dean