tranny input shaft question

i have a 1990 XJ with a 4.0L, AX-15, and a np231. now im in the need of a AW4 and found one and its a 21 spline. i know that my AX-15 t-case is a 23 spline but im not possitive on the input shaft that goes in the block. is there any difference between the 2. the AW4 im getting is out of a 89. will it fit. any info will really help me.

Thanx guys
Paul