You have to examine the "wavy" and see if splines are still thin as others or that grooves are as wide as others. If it only takes a gentle filing/sanding and it's localized, no problem. You will have to try it anyway before buying another. Make sure you are not experiencing "hydraulic lock" with grease sealing it, or hole plugged or too much grease down in there. Also be sure it is not "keyed" (one larger spline, etc). This is sometimes done to make sure the U-joints are phased.
I guess you saying the female splines are wavy, what about the male? In other words the driveshaft. Are they OK?
