They both fit the rwd FX. When I ordered my Timken, I order the rwd version, but the awd with splines showed up. No issue installing them. But, one did not on last long... Approx 10,000 miles. Got it replaced under warranty and they sent me the rwd unit. Been going strong since.. Approx 20,000 miles on these.
Also, I've seen some rwd units have a cap located behind it covering the axle hole, and then some don't. If I remember right, the oem axle comes with the cover installed.. The Timken has no cover.