That 6818 seal should be the right one. The GM PN from the attached catalog listing crosses over to Timken 6818.
As far as marking the u-joint flange, you need to reinstall the driveshaft in the same position as it came out to maintain the balance. Also the shop manual recommends marking the pinion flange so it's reinstalled in the same position on the pinion shaft. Mark both the pinion and the end of the pinion shaft. Check
HERE for the complete procedure. (That's from the 55 second series manual, but the rear axle for the 55.1 is the same as the 55.2) That includes checking bearing preload before disassembly and rechecking while tightening the pinion nut so the preload on the pinion bearings is correct. You can't get that right by marking the position of the pinion nut before you remove it. It's important that the bearing preload is right, or you may get gear whine or early bearing failure.
