postindex is used for searching. You can remove that and then rebuild your search indexes after you import/install the forum. But, the size of that table is smaller than your post table. I'm surprised you are having problems with it as it isn't that large. What method are you using to save it and import it?
From the manual:
Backing Up The Database via SSH/Telnet or
Backing Up The Database via phpMyAdmin
Restoring The Database via SSH/Telnet or
Restoring The Database via phpMyAdmin
If you don't have shell access, some people have also reported success with these scripts:
MySQLDumper
MySQLHotxcopy
Bigdump