::EDIT::
Same problem with my car. I backed up traffic durning rush hour.
Do what the others suggested. Check to see if the linkage is loose in the shifter (I too have a B@M shifter).
If not, it could be the bolt head in the shifter tower on the trans sheering off. (mtx's with the cable shift) I've seen pics from Terry, of the bolt sheered off and getting stuck in the trans gears then blowing out pieces trans. i was lucky i was stopped when the head deceided to come off.
New shift tower is like 300 bucks, the bolt if you can extract and if given no damage to the trans is like 5 bucks (wierd bolt size M7 x 1.25, about 30mm length) If you done work on your car before you could possibly do this in your garage. (what i did) If not take it to shop and have them check and contact Terry to see what he can do to help. IF there is damage to the trans and needs it repaired or upgraded, Terry can help!
Hope this helps
::EDIT::
dang responses are quick.. it looks like the cables are fine. it's prob.. the bolt head that sheared off. I DON'T think the trans needs to come out to see if the bolt has been sheered off. if the mechanic can take the shifter tower off. (under intake and trans mount and @5 8mm bolts on the shift tower) he can see on the shift tower rod to see if the bolt is sheered of and also see if the trans gears are intact. Cuz what he quoted is alot of money.
