I presume you have an automatic, 'cause you didn't say so in your post. Slushboxes fail and that's life. usually they fail in drive and only the reverse works so maybe you got lucky. Check that all the shift linkages are up to spec. that nothing is binding that any vacumm hoses that manage the system are in order, have a look at your tranny fluid is it black (bad!) or pinkish (good!). It really sounds like at least the torque converter is still good (drive works) so you might not have to pull the box. If you are interested in keeping this car for a long time I would consider modifying it to a manual. I don't think you'll find the manual stuff for a V6 in N.A. but even with shipping a used manual set up from overseas won't be much more/= than a rebuilt auto box. Don't bother switching in a used auto., that's just a waste of $.
BTW I'm presuming that the V6 is available overseas with manual.