I recently did a Front Main Seal (front crankshaft seal, timing cover crankshaft seal) replacement on my 1994 ford ranger 4.0 Automatic w/ac.
I ran into problems with the front main seal provided by Rockauto. I purchased what was listed as the 'front crankshaft seal'. It was a timken 2012V. This seal was about 1mm too small to actually seat in the timing cover.
Additionally, many sites list the SKF W0133-1816662 as the correct seal. This seal has the same O/D spec as the Timken 2012V and most likely will not work.
Long story short, Napa had the correct seal, the SKF 16473
I did a brief write-up with more details and pictures here
Ford Ranger - Useful-Info