to check the center bering just see if there is any play in the inner metal sleve. its a sealed bering so thier so thier shouldn't be any gritty grind feeling when you turn it. also, thier is a rubber vibration grommet the goes around the bering, replace it if its cracked, brittle ect.
i think the reason my previous techs didn't want to replace it is because they had to take the drive shaft out. lazy

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320