Don't panic. The individual parts are supposedly balanced at manufacture, rather than as an assembly, therefore the refitting in the original position is just a safeguard against any wear or settling into position and effects the preload as much as the balance, I seriously doubt it would casue that much vibration and would have been evident from day 1. the more likely possibility is that the assembly has become loose, that could easliy casue the symptoms you have described. Did you tighten everything back up and fit a split pin? if so then it should all have stayed put. Obvioulsly something is amiss but you really need the car home to sort it yourself. The diff I had collapse had recently been overhauled, I've learned not to trust any mechanic I wouldn't go for a drink with ie my mate or myself.
