2002 SANTA SHAKES AT 40 MPH

Your front wheels are out of alignment and probably tires out of balance. Go to a service station/garage. Should not cost a great deal.

Tires should be balanced and rotated every 12,000 miles (approximately every four oil changes) to ensure even tire wear and extend tread life. Out-of-round means the tire has worn unevenly and cannot be balanced. An out-of-round tire will have to be replaced.

Its the problem of speedometer programming.

OR

You probably have mud caked on the inside of your rims and on your suspension

OR
I had this problem also where my car would shake between 55-75. It ran smoothly at 80 (don't tell the CHP). Everyone I asked said tires, so I had the tires balanced and realigned, but that didn't fix it. Most of the mechanics I asked were stumped. Finally someone recommended having the struts looked at and sure enough, I had the struts replaced (the car had 70,000 miles on it) and it worked. I have never had this problem since (now the car has 150,000). Answer I have sold tires for a number of years and I can assure you sometimes new tires can be out-of-round. Take it back and demand that this problem be resolved.

Check the MAF sensor, test the fuel pressure at speed problem occurs, do a live data test while driving and have someone look at the data to see if they can see what is happening, this could also be many other issues, your only real choice is to take it to a dealer for in depth diagnosis, they have the electrical test equip to do the testing as well as a huge data base to draw on to find the problems, you have been buying allot of parts to try and fix this, do yourself a favor and let the dealer tech's locate the problem