I had a similar experience with mine. What I finally found after removing the carrier bearing was very fine pieces of gravel almos sand size had got between a small metal sheld made on the carrier bearing and the drive shaft. Removed the carrier cleaned everything and reinstalled the bearing. As for the fix your dealer spoke of instead of a screw I installed a grease zero in the yoke that splines on the shaft. To insure grease could get all the way around the splines. I took a dremel tool with a very small cutoff wheel and cut a groove in line with the zero all the way around the inside of the yoke. Now if I start feeling a vibration I can access the zero from the area wher the shaft connects to the front differential. It’s hard to reach but I can get it in a couple minutes without taking anything apart.