had that happen to me at low speed, put a tow rope on the car got my mate to give it a good tug with his car and shot a cog out the box which freed the gearbox up and he towed me home

fiesta box has no lsd, diesel box has no lsd and is no stronger
best bet is to get a std recon one if ur running sub 180 bhp anymore than that then go for a stg1 bernie box or one similar