Use DBCC shrinkfile with the EMPTYFILE option to migrate all data from the specified filegroup.
This will migrates all data from the specified file to other files in the same filegroup. The SQL Server 2005 Database Engine does not allow for data to be placed on the file used with the EMPTYFILE option. This option enables the file to be dropped by using alter database.
Just in case.....test with a temporary database first.
Eric