Well to tell you the truth I'm not 100% sure but I think all the auto's on the Jeeps have the 23 spline.
I know the 207 behind the BA10/5 for 87 has the spline count at 21.
This said you should be able to bolt the 231 in place of the 207 behind an auto. Spline count is the only issue as bolt up is identicle.
Hope this has helped some.
The bottom line is spline count 21 on NP207 and 23 on NP231, auto is the exception,,,,, I think.