instead of going to all the trouble of swapping to 23 spline.. I say leave it 21 spline. Obviously its holding up well thus far right? IF for some reason you bust a shaft.. Longfield makes a chromoly replacement which should stand up to the beating.
I've been running 21 spline dual 2.28's for around 4 years now and havn't had a problem. I'm going to be in the same boat as you soon though because the rear output shaft leaks like crazy and I have a feeling it might be a bearing.
