Geographic mobility of workers within Germany, as compared to within the US?
Here is a figure documenting annual (I think) internal migration rates in Germany:


Internal Migration in Germany, 1995-2010: New Insights into East-West Migration and Re-urbanisation  (Sander (2014)) 

This table show 5 year migration rates in many countries, including the USA:


Cross-national comparisons of internal migration: An update on global patterns and trends (United Nations Department of Economic and Social Affairs (2013)) 

Depending on methodology, annual and 5-year rates can't exactly be compared because some people will move to another area and move back within five years, so asking people if they live in the same state as 5 years ago won't give the same result as asking someone if they lived in a different state last year for five years. That said, if we can make some rough comparisons if we treat the five year rate as roughly comparable to five times the annual rate. The USA rate is about 9 percent every 5 years or 1.8 percent per year. This would be higher (but not much higher) than the West-East, East-East, and East-West German internal migration rates but quite a bit lower than the internal migration rate within West Germany. It might be that this converts the 5-year rates to annual ones that are too low, but the sizes seem roughly comparable. This is just two sources but they provide little or no evidence for Feldstein's claim that the German rates are far lower than in the US.