I have a 99 expedition I had the tires changed I put smaller tires on it to get rid of the mud tires now my front end shakes like it has 2 flats

More than likely there is some component damage ,linkages, tire rod ends, vibration dampener the shock looking thing under front end , connecting left to right on steering link.

I've worked for tire centers before. This was a common occurrence that I had the pleasure of helping resolve many times.
The tires and wheels both have high and low points. This is the reason tires and wheels have to be balanced properly. However, adding weights only alleviates some of the problems. The installer should have done a Static Balance. That's where the tire should have been installed using the High-Low balance. The high point of the tire with the low point of the rim/wheel. This reduce the amount of imbalance right from the start. The last thing is a Road Force Balance test. This goes a little beyond the norm of balancing. It simulates the imbalance with applicable road force to help determine what the balance is at operating weight. for some vehicles, it's not important ( small cars and small trucks ) . In the off chance that none of this works, you're probably looking at a defect in the new tire. This is very rare, or the off chance that the wheel was bent, and the larger chunkier tire dampened the imbalance.
I do hope this has helped in some way