[Ramblin Fever]

Is your undercarriage coated with ATF? Any drips on the ground?

There is always the possibility of ATF getting out of any hole - I just replaced a $6 seal - shift shaft seal specifically, that's been slowly weeping enough fluid over the course of the last few months to cost me to lose about a 1/2qt a week. If I'd have left alone without consistently checking and refilling the level, I surely would've cost a lot of damage.

These 4L30-E GM transmissions do not like to be low on fluid, so I would recheck the level ASAP - I doubt the missing bolt alone would cause a vibration, but it's possible if you're low on ATF. That little pan holds about a quart of ATF; but you'll have to double check your level.

Not sure if you have a dipstick or not on that particular year - if not, your refill hole is on the big transmission pan in the back corner, the highest bolt on the passanger side. Search through the archives on the correct way to check/fill this fill-plug, don't attempt it without reading up on it, or you may be swimming in a red bath.

Not exactly sure if your vibration has anything to do with the tranny or not; are you having any other symptoms?