block error, but can't pg_dump

I have a table that reports one block error when I try to select
everything, reindex, vacuum, etc. (invalid page header in block
413620 of relation "tablename")  I've read on the list that the thing
to do is use pg_dump and then restore it.  However, pg_dump fails
with the same error.

I can't take down the whole server, let alone easily Postgresql, to
debug this.  Any ideas as to how I could do this?  I wrote a Perl
script to select the records one by one but it didn't find a bad
record although maybe it's how I wrote the script (it just selected
the records one by one.)

I also don't know enough about Postgresql data/blocks to figure out
which data is in that block and what I'd do with that.

Thanks in advance.  P.S. "relation" means the table, not an index,
right?  I don't see an index with the same name as the table, but
thought I'd check.