Could be the 'worm' drive cog on the mainshaft in the gearbox,mine fooked up when my speedo cable melted on manifold

Gearbox out and split the backend off gearbox if it is this thats gone

Not available from Ford

As said above,remove speedo drive first and check that,if that seems ok stick your finger in through speedo drive whole and feel if the worm drive cog is there,and if its solid(attached to shaft),if it spins around the shaft like mine did its gearbox out time m8!!!!!!!!!