Grant,
The speedometer drive gear, in the gearbox, is matched to the rear differential, but usefully is colour-coded.
The part is #52 in the drawing but you need the correct colour since the speedometer will read incorrectly otherwise.
http://www.oemepc.com/vw/part_single/catalog/vw/markt/RDW/modell/LT/year/1985/drive_standart/6/hg_ug/311/subcategory/47010/part_id/3690672/lang/e
Use the Buyers Guide in the menu to the left, to ascertain if the part is still available from VW. 

https://www.brickwerks.co.uk/lt-parts/drivetrain/lt-gearbox-transmission.html
A few in stock now.