On a 2005 Taurus, I upgraded from 16" to 18" rims. Now the rears are shaking badly when I get up to around 40mph. What could cause this? This is a 2005 so I'm assuming the struts arent worn to the point that it would cause tire wear that bad and I never noticed it before changing the rims. Is this most commonly a balancing issue? Any help would be appreciated.

I took some info from your other post.. Are these also called spacers, by another name??? They're like 5mm or so thick??? I see from your other post that it's a noticeable wobble, also causing the steer wheel to shake,, seems you have a major issue going on, was the clearance with the wheel and hub checked?? Something's probably not installed square, or bent, maybe too many spacers with stock lugs??? I'm not familiar with putting bigger then stock wheels on newer cars, 18 from 16 seems a rather large jump, imo... I thought wheel spacers were illegal, don't take that to the bank though, I've been wrong plenty, If you changed wheels and these problems cropped up, then they're the deal.

Did you balance the rims? They might need some counterweights. Another possibility, you have a tire getting ready to blow out. Even if your tires are fairly new, you still can't rule out a defect.

I suggest that you check them for runout. A lot of aftermarket wheels are made for appearance and aren't straight.

95% chance that the tires just aren't properly balanced

Were these new or used rims? If you rotate front to rear, does it make a difference?

I'd say take them to get balanced before you do anything major. Balancing is cheap and unbalanced wheels, especially four can be pretty bad.