The snubber should help the pinion angle stay in a safe area if the rear springs are overcome by the weight transfer of the car. Many say no pinion snubber is needed however I am of the opinion that if you have to have soft springs to get the tires planted a pinion snubber will be very helpful in keeping the pinion angle safe. What springs are in the car now? I would def. have the drive shaft balanced by a known good shop with the new u-joints installed. That said let them put the joints in if you do not have the proper tools or experience. If this is the case as if you can watch. I've seen several shafts ruined by inexperienced, overzealous shadetree mechanics. At the age of 16 I started that list! If you have trouble getting a response shoot me a PM or I'd be hapy to share my experiences.