You do not need to upgrade the splines in the rear case. The 'Total Spline' system is quite strong even in 21 spline.

In fact, I run the 21 spline in the rear case even though the Chili sports a 3.4. You will never hear them claim that is recommended, but... they have never had a failure in the TS system to date (last I checked).

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell