I assume you are talking about a GMC but no mention of what the vehicle is.
That said, I cannot see why you are having trouble UNLESS the seal is the wrong part and the spline cannot fit inside the inside diameter of the seal. The drive shaft spline is not keyed so it can install at any position. The reason you mark the shaft and both mounting surfaces is so that the drive shaft is put back in the exact same position, its call phasing. If its not you can often have balance problems because you could be 180 deg out.
If this is a 4 wheel drive truck you do not want to put grease on the splines, should only use ATF to lubricate.
