it would be side-to-side movement you are looking for. go ahead and give the driveshaft a good jerk. im shure you will find your problem. the flex disc in the front, after the trans is usually the one that goes bad first. there is also a center support bering that should also be checked.
when i reciently replaced my flex disc the mechanics said my bering didn't need to be replaced. i think they are liars. i don't do buisness with them anymore.
anyway, im not a tech but thats what i see from my experiance. hope this helps!
good luck,
adam

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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