Possibilities are endless. If it was sudden, could be a flex disc. Unless you hit something, it's unlikely that it's a tire thing. Negative on the oil change. Could be engine related as well. Coinciding with you gear change scenario. If you're not an avid DYI person, baset get it checked by someone you trust...Good Luck.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.