I'm not sure how or why this would have been caused by replacing an axle unless whomever performed the replacement is a 400lb Gorilla named Mungo, but since you DO have a mushy clutch pedal I'd recommend that you bleed your slave cylinder. It's very easy to do. I bled mine when I replaced my slave cylinder during my LSD install.
Remove the airbox and search around under the foam-wrapped wire harness here:
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Then pop off this yellow cap and attach the line from your gravity bleeder or vacuum bleeder, and twist the valve open:
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Make sure your brake fluid reservoir is topped off and give the clutch about five pumps, holding it for a few seconds each time. While bleeding the clutch pedal will bottom out easily and you may need to pull it up manually - this is normal. Confirm that there is ONLY fluid in the line attached to the bleeder valve. Twist the valve closed, remove the bleeder line, and replace the cap. Your clutch should now be nice and firm.
If this doesn't solve your issue, my best guess is that Mungo busted something. That boy doesn't know his own strength.