I think the spline wear was caused by "tired" o rings inside the mid shaft. Once they are worn- they become sort of flat at the shaft contact surface, - all the grease you might have put on the new drive shaft, washes away rapidly. Then, rapid wear occurs. Keep in mind, whenever you change pumps, you should remove the midshaft assy. to check for proper pump alignment- good time to replace those 2 o rings then.