Yah, D300 are all 23 splines. The late model years will have a longer output but it is nothing much. For use as a doubler, it is a good plan. Choose your doubler case to match your tranny output and the input to the D300. If you have a 5-sp trans, you will have a 21-spline output. But if you use that case as the doubler case, the stub shaft between it and the D300 should be the same as a 23-spline NP231.
You will have to flip the D300 as well. So you may have to rebuild with new gaskets, relocate your breather, reconfigure your shifters, add a cv style yoke to the output, and get a speedo convert kit to work with your TJ. It is all good. I've got a D&D Machine kit in the garage that requires some assembly and a D300 as well that needs the rebuild.