i have a 2000 f250 7.3 with 35in tires. but when i get between 40 and 55 mph my wheels shake rapidly back and forth as if im jerking the wheel side to side what is causing this???

 

I would check the front tyres for any bulges or distortion. Check also that the rims are good and not bent in any way. If all that checks out it sounds like you need the front wheels balanced.

 

at that speed range, sounds more like balance. First, make sure the inside of the wheels are clean, no mud, foreign objects stuck in there like a Toyota prius, etc...
Second. make sure the tires are straight ahead and get some twine, pick a spot on the tread toward the front that will have an exact copy on the other side. cut string to exact, then measure the same spot on the backside of the tire. If it is the exact same, move on to next suggestion.
Third, Jack up the front end, set it down on some stands (safety first, middle and last), spin tire as fast as you can by hand and look to see if there is rim warpage. If you think there might be a slight warp (thats all it takes), spin the wheel again and take a small dowel, piece of a twig, or something that won't scratch the rim, and hold next to rim edge. If it stays on rim, its straight.
Third, take to a tire dealer that has knowledge of oversized tires to be re balanced properly.
Fourth. If balancing still does not alleviate problem or only lessons it, Find a truck dealer (Kenworth, Peterbilt, International, etc..) That can shave the tires.
We had balance problem on a set of steer tires on a kenworth that we couldn't balance right, we took it to a Mack dealer who shaved the tires then rebalanced them. We have not had a problem with them for the last 100,000 miles.

 

thank you upon looking better it appers that the left tire is turned outward slightly i dont know if this is my eyes or it is truly this way. i was also wondering if i would benefit from steering stabailzers