I would say a transformer should not considered as a first solution. There is a reason why you are getting voltage drop and it would be best to discover why before comitting to anything.
First off at which point on the connection did you take your measurement? Lugs in panels? At the top of your mast (if you have an overhead service)? What phase is showing the voltage drop? did you test both phases to ground than phase to phase? What were the results of that?
With that information, a good picture of the problem could start to be formed.