just had the same problem with an 04 transit stuck in 5th gear. after removing both shift cables, wiring block, breather pipe & 6 retaining bolts I pulled out the selector box expecting to see the 10mm bolt either loose, sheared off or missing altogether, but to my surprise was still firmly in place. problem turned out to be the rubber seal at the top of the input shaft allowing water in & the shaft had a patina of rust on it. after freeing with wd40 I gave it a good dose of duck oil, worked it vigorously til the movement was free & easy, applied a generous blob of grease to the protruding part of the shaft & replaced. customer said the gearbox felt the best it had ever been & all it cost was a couple of hours labour. so dont be too disheartened, it might be better than you think
MOBILE MECHANIX, December 2012