Vibration in the drive train getting worse.

put tires from rear up front and front to rear and see if tires cause a shake also make sure no debris on hub so rim sits flush if no change and still vibration check drive shaft and u joints

Have the wheel bearings checked.

A couple of things to check. It may be a wheel bearing, shock, cv joint, or a very common cause is that one or both of your back tires are worn irregularly. This causes the car to shimmy and get progressively worse as you have explained. It usually happens on the front end of the car, but can occur in the back.

this could be a few things. if the odor was of rotton eggs...it's either a broken/loose motor mount that shakes the exhaust system causing the catalyitic converter to make that smell, or, in a worse case, its your transmission overheating and burning up ATF (automatic transmission fluid) and this is causing the torque converter plates to rotate out of harmony with each other causing the vibration. if the smell is more a burnt rubber, it's much more likely you have a timing issue or a worn main bearing on your crankshaft. this causes vibration and wear on the belts causing them to "burn" or emit that burnout smell. either way, take it to your mechanic right away because none of these problems will end well if left unattended. good luck!

may have a bad tire you can balance a bad tire but will still shake

If you get a bad vibration in your steering wheel at low speeds without braking it is either a bad tire or a bad rim. If you get a vibration at speeds around 45-50+ and goes away you probably have a balance issue. If the vibration only occurs while braking then your rotors are warped. If low speeds it shakes i'd take it to a shop to check your tires.

I would guess incorrectly tightened or installed wheel bearing. Or it might just need to get greaced.

Check the U Joints. My wifes xterra had 80K miles on it, same symptoms. Was a bad from u joint. Nissan mechanics and 1 tire shop could not figure it out.

Wow..thats not good..first have motor mounts checked, maybe the tranny mount or possibly if ur car has it is the driveshaft bearings...good luck on the oil leak...maybe tranny fluid..make sure if ur losing fluid that ur checking them incl. the tranny fluid...u might find ur leak first of all by rechecking the oil cap where it is supposed to be ...same with all fluid caps.