Q: Shuddering with antitheft light

I have a 2005 mercury mariner. Recently I have had am issue with highway driving. Around 40mph it shakes and around 50 it shudders. The shaking at 40 is brief. The shuddering at 50 is sometimes brief (and I can continue to accelerate to 60) or it sometimes lasts longer and then my antitheft light comes on. My antitheft light also comes on if I try to accelerate past 60. The light stays on for about 6 seconds and then goes away. If I hit the brake or pull my foot off the gas and Coast it might go away sooner. I am the only owner, I have the only key, it is the same key I have had since buying the car.

My car has 180000 miles.
My car has an automatic transmission.