The car shakes when driving over 50mph. tires have been balanced, rotated and the alignment done. the check engine light came on when it first started to do this and has since gone off.

Needs a front end alignment

Because one or both of the front wheels are out of balance.
1. You need to have the front wheel/tire package rebalanced by a competent balancer. Go to a good tire retailer. They have the equipment to do the re-balancing for you. Don't go to the cheapest shop you can find and hope they will do a great balance job for you.
2. If you still have the vibration after re-balancing (assuming it is done well) you will either have a faulty tire or tires (out of round or a heavy spot in the tire) or there is a problem with one or both of the wheels. If either of the front wheels have been slightly bent out of shape from abuse you will get a vibration in the front end of the car and in that event the wheel needs to be replaced. If you have a cheap brand of tires fitted to the car and the re-balancing does not solve the vibration problem I would suspect the tires first. Cheap tires will typically give you problems like this if you buy them but better brands can have tire faults also.

3. Do not let someone tell you that a front wheel alignment will solve a balance problem with the wheels/tires. It will not because the vibration is caused by the rotating wheel and tire. By all means have a wheel alignment done again - because you need to do this on the car about every 10-12000 miles in any event - but doing this will not solve a balance issue.

4. If you want to run a test before doing any work have the rear wheels on both sides moved to the front of the car and the fronts to the rear. Then check if you still have exactly the same vibration. If not then you know where the problem is. If you still get exactly the same vibration or a similar front vibration with the rear wheels fitted on the front, then the balance on the rear wheels/tires will need to be checked and re-done as well.

Being a mechanic try to have tires rotated first to see if its a rim issue or broken cord in tire if balancing rim don't work an rotation don't solve that you may have 2 things that may cause this. CV axle bad or warped rotor

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have an inspection done on the front end to determine for worn failing parts in the suspension and steering. Then have a wheel alignment done as what you have is a castor wobble and it is serious .

Start by checking the tie rod ends on both sides on the front wheels..With the truck jacked up and jack stands underneath the frame both left and right wheels should be suspended in mid air..The are two tie rod ends on each side of the vehicle, a inside and outside tie rod..Check for any excessive place on each one...
Also ckeck the balance on all four tire's... Inspect the wheels for any stress cracks that would cause them to not to balance correctly and make your vehicle shake...
Check the transmission mounts on the cross member,make sure the mount is secure to the Trans and cross member mount..Also check both u-joints on the drive shaft and any hanger bearings that may be underneath there..If it's a 4 wheel drive check both drive shaft's...Anyone of these option's could show those
symtom's you mentioned...Good luck and let me know how it works out..

sounds like perhaps one of the drive shafts could be bent? has someone supported the vehicle in the wrong place or the jack/ramp or the car has slipped off the supports you need to get that checked out

Front end alignment will fix this. Any time I have my front end aligned, I also have the tires rotated and balanced. Get your tires rotated and balanced, and get the front end aligned. Not too costly and you'll be amazed in how smooth it will ride after you have it done! Easy fix.

good luck!

Have the alignment checked on the front end.

check ur suspension

Anti-lock brakes mean the car shakes when the brake is applied. Try pumping the brakes.