I thought about it some more and remember I used YJ slip yokes in my XJ driveshafts, before finally going SYE. The YJ yokes allow a bigger angle because the ears are longer, so even though the diameter and spline count match, you might end up with some yoke binding.
ZJ, XJ, YJ, MJ all share the same output shaft spline counts UNTIL 1996 when ZJ's and XJ's went to a sealed output shaft. Also, the YJ slip yoke is about 1-1.5" longer than the others.
Make sense?