I have a 97 s320 and i get a vibration between 50-55mph after that it smooths out

Check the tyre tread by running your hand over to feel for any high or low points. This is an uneven wear pattern caused by worn shock absorbers or struts and usually is felt at speeds over 45 mph. You could rotate the tyres depending on their condition. If the vibration is felt through the steering wheel the front wheels may need a balance but this is usually comes and goes well below 50 mph.

The 1st thing to get looked at is the wheel balance.Have them checked.At the same time,if the tyres are o.k. have them rotated.This means you swap the tyres from the front to the back.Always keep the tyres on the same side of the car.So left rear goes to left front and the same on the righthand side.Also,a wheel alignment should be done.These are a tyre shop job.Cheap and doesn't take that long.If these don't correct the problem,the next thing to get looked at are the front wheel bearings.Get the tyres done 1st.This is the 1st step and also the most common problem.Hope this helps