Downshifting to 3rd while at ~60 will not redline these. With 3.42 GU6 RPO, you can reach 55 in second gear and have a few MPH till the shift point, IIRC

 

Not sure if no one knows how to fix them, or they aren't worth fixing, gotta be one of those options

 

I had a beater 2001 Ford Focus I use to go back and forth to college with it got great gas mileage. It did do all kind of strange things but it ran..the windows gears were made of plastic so if you held the button down when lower or raising a window you just blew the gears and that was at 100 bucks a pop to fix your self and that was junk yard prices if you could find them...in short supply....the a/c did work it just took awhile to cool things down with those big windshields that let all the sun light in. I would like miss one time going down the road then run okay again....The person I got it off was a women and I don't know who did the work on the car for her for I think they were trying to bump her off...Brake parts missing, cal held on by one Harley type bolt, brake shoes were down to the metal at an angle...car was a trip, no spare tire or jack..good thing I got it on the cheap......traded it for a chevy.....

 

It could be a separated tire. It makes a noise like a wheel bearing hum and can cause a vibration. If you had a friend with a good set of tires you could swap out all four for a drive to see if the vibration goes away. If not, then try using your spare and swap out the spare for each of the four tires to see if the vibration goes away. A little work, but worth it to find out free without changing any expensive parts unnecessarily.

 

^ + 1
Check lube diff front and rear.