Mysql SHOW OPEN TABLES shows In_use but nothing in SHOW PROCESSLIST
Speculating, since the tables are in MyISAM, perhaps there is a corruption in the table(s). Since it is locking when checking with the server online, try using myisamchk on the stuck table(s) (and perhaps all the tables) after shutting down the server if possible. 51GB is a lot of data so it may take a while. If possible, suggest following commenter's recommendation to convert the tables to InnoDB; adjust server configuration accordingly.

Ref: http://dev.mysql.com/doc/en/myisamchk.html