shouldnt need new splines unless you have insane amount of flex. mine twists up pretty good and the xj shaft shortened worked fine. if it doesnt have enough travel, you'll need a custom shaft built.
to cut down my xj shaft it seems like it was around 50 bucks. i got both my shafts done at drive shaft shops.
'90 YJ, ford 8.8, SOA, lincoln front, aussie rear, 4.88s, swampers.