A 90 Yj with a 4.2L has a Puegot BA10/5spd behind it. You can tell this from looking under it. the BA10 has a seam running along the bottom of its length, with bolts holding it together.
An AX15 has a flat bottom, with a drain plug, all aluminum. AN AX5 has a flat bottom but the piece that bolts to the t-case is cast iron and probably rusty. That's the easiest way to tell the differences in the trans.
The bell housing for a BA10 will accept the AX15 as long as you keep the internal throwout bearing, but does the AX15 Have the cone on the front of it for an internal throwout? Plus, if its a newer AX15 the nose of the shaft is .75" in diameter and the BA10 has a nose diameter of .565" I think, just over half inch. So a new pilot shaft bearing is needed, in the back of the crank.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...