From the description I would say that something has jammed the transmission between the gearbox and the differential. The gearbox and back end are heavy duty work. Gearbox removal will need, cab lifted, several jacks, three good men. A complete box from a breakers yard could cost several hundred pounds.
Drain some oil and check for metal.
Best case, the gear selector mechanism has jumped two gears in at once (unlikely). Remove the top of the box and check for damage.
Jmmy
November 2009