Sticking in 2nd!!!
At ALL RPM's, or just higher RPM's?? Is it also difficult in going into 4th??
If just 3rd, then probaly synchro and/or 3rd gear itself (engagement teeth)
but can still be a key or fork issue (even though, if that was teh case, you would probaly have a hard time getting it into 4th as well, since they share teh same keys/fork)
Either way, does not sound like major surgery though, even though the tranny is gonna have to be opened up to fix it.
Keep in mind, that the longer you drive with a problem, teh possibilty of damaging something else can occur.

