[Bret Brinkmann]

Here is what I got out of JDM CAPS. With the change from 18 to 25 spline output shafts they also changed the VCU, sun gear, and pinion shaft, bell housing, and output shaft seal. If you switch from an 18 spline to a 25 spline you will need the follow parts.

25 spline output shaft
25 spline output shaft seal
25 spline bell housing
25 spline sun gear
25 spline VCU
25 spline pinion shaft

sipe16 ws correct about the differences in the VCU, sun gear, and pinion shaft. I'm working on pics but probably won't be able to post them until I get home. (Away from home for 2 weeks for work related training.) But in a nut shell the diameter of both the pinion shaft and sun gear are smaller for the 18 spline. Also, the placement of the splines in the VCUs are different. The 18 spline has the spline more towards the pinion shaft while the 25 spline has them more towards the sun gear. With the defference in VCU spline placement the 18 spline has a taller sun gear and shorter pinion shaft than the 25 spline. The planetary carier and center diff housing are the same through out the years.

NOTE: You can use an 18 spline output shaft with the 25 spline VCU but you HAVE to use the 25 spline sun gear AND 25 spline pinion shaft. If you change either one of those three you HAVE to change to other two also. Obviously you can't use a 25 spline output shaft with the 18 spline VCU/sun gear/pinion shaft because of clearance issues. Also, the 5 and 6 speed 25 spline transaxles have different pinion shafts and bell housings (among other things) but they use the same output shaft, output shaft seal, VCU, and sun gear.