The AX15 is a direct bolt in trans. However, there are two issues you need to research beforehand.
First, look for an AX15 from a 94+ XJ. These came with the external slave cylinder. You could get an earlier model and change the bellhousing and internals.
Second, the t-case that mates to the BA10 has a 21 spline input shaft. The AX15 has a 23 spline output shafts. So, you will need a matching t-case to go with AX15, or swap the input shaft on the t-case.
Additionally, as thejim42 mentioned, the crossmember for the AX15 is differnet from the BA10.
To recap:
1. Get a 94+ AX15 with bellhousing.
2. Get matching t-case (or change input shaft)
3. Get AX15 crossmember.