^^^ What he said. If you have a standard (non STi) gearbox in your WRX now (pre 2002) try to find a late model 6 speed so you don't have to play with the rear diff. If you have a 4.44 ratio 'box now you will have to change the rear diff anyway.

The box that has jammed in second can probably be repaired fairly easily, but it'll still cost you in the region of at least a thou by the time you pull it, split it, assemble it and refit it.