The inner drivers connect the transmission to the drive shafts. If they have a lubrication failure, or just from wear and tear, the thrust surfaces can develop depressions such as those highlighted above. A very small depression can cause a big vibration problem under loads or acceleration as the tripod bearings rides in and out of the worn area. Sometimes swapping the drivers side to side is a viable alternative to replacement. If longevity is desired, replace worn inner drivers and the tripod bearings (splined to the drive shaft) and inspect the boot's condition. We have had longer life from them using Castrol Wheel Bearing Grease NLGI ( a lithium based grease). Right side failure is more common than left, probably due to the exhaust heat's effects on the lubricant and boot.