Yeah I'd be lucking at the selector arrangement under the tunnel, the stick engages in an array of slots(you can see oall of this on top of the 'box when you remove the centre tunnel cover), the end of the selector lever may have worn and got itself stuck inbetween 2 of the selectors, jamming the box in third.
This should not be so hard to fix, if its not this though then you've got a bit of work to do as its a 'box out job, which on a lannie is a big job.