I have sierra axle that i think came from a 2.0L 4x4 sierra. don't know what year car was but it has discs fitted. Think its about time i changed wheel bearings.

When you remove the bearing the part number will be stamped on the outer race when you have the number go buy new one

Just bought a new one for the F27 front outer (Escort) from Whych bearings as they seemed to be the only people with the same refs as on the outer track.
linky
Fast service and very good prices.
Recommended.
HTH.
Cheers, Pewe10

^^ all the 4x4 sierra's use the same rear bearings, also the same part as the mk3 granada's and 2wd sierra's with ABS (not cossie's though)

http://www.burtonpower.com/wheel-brg-kit-r-sierra-cosworth-xr4x4-abs-granada-mk3-brt910.html

you should be able to order one from any motor factor, they don't all stock them for such old cars these days