I've never even heard of an NP-231AMC, what is it out of? The bolt pattern would be the same, but you should definately check spline count. It is very possibly a 21-spline unit. Also, make sure that it isn't an NP-207. They look very similar but the 207 is much weaker.
Pretty cool... It's the same as an NP-231J from '90 or newer Jeeps. '88-89 used 21-splined input shafts. '84-87 Cherokees and Wranglers used 21-spline NP-207 x-cases.
I've heard that all automatics used 23-spline shafts.
So...what year is your Wrangler and which transmission does it have?