We need to restore some old content from an Exchange Server 5.5 server's backup tape. Can I restore priv.edb from tape and load that file into an Exchange Server 2003 Recovery Storage Group?

A Recovery Storage Group can mount databases only from Exchange 2000 Server Service Pack 3 (SP3) or later. But here's what you can do: Create a database on the Exchange 2003 server, dismount it, throw away the database's .edb and .stm files, and drop in the Exchange 5.5 database. Give that database the same name as the database file you threw away, then try to mount it. The Store will create an .stm file and the metadata necessary to mount the database, converting the on-disk structure as necessary. You can then attempt to mount that database in the Recovery Storage Group.