Looking at the Sierra manual there seems to be two alternatives the newer nylock style with left and right hand threads and it suggests older vehicles used conventional nuts with a 'stakeing' ring to lock the nut once tightened.

Inspecting my drive shafts I have the latter with clockwise tightening thread both sides and the stakeing indent at the end of the shaft.

I've seen plenty nylock style nuts but where can I get the old type from, does anyone have some surplus to requirements?

I had this problem once, but got round it by fitting the nuts on the opposite shaft and after torqueing them up the part of the staking rim adjacent to the slot was thankfully still virgin metal so they were reusable.

Otherwise you will be able to get them from a Ford dealer.