I came home from work and noticed that the shock on my front, passenger side was leaking. I replaced the whole strut assembly. Do I need to replace the other side as well? What will happen if I don’t? My truck is a 2005 F150.

I would, you want both sides of the front suspension to react the same over bumps and curves. Odd handling could otherwise result, not a good thing.

+1 to texases’ recommendation.
On a 9 year old vehicle both struts have gotten a lot of wear, and in addition to the probability that the other strut will also start leaking soon, the immediate problem would be a ride that is very much unbalanced, and handling that could be…unpredictable.

Agree, plus you need to get an alignment anyway. Why do it twice when the other side goes which may not be far behind.

+2 to Texases comment.
The others made good points as well.

Contrary view: Pay close attention to how your truck reacts to suspension motions. Go to a parking lot and do some quick left-right zigs and zags, with and without braking. Any difference? How about going over straight-ahead bumps - any difference in how each front corner reacts? If no difference, I see no need. I had a dealer replace one front strut on a car once. He said it’d be fine, and it was.

Your truck is 10 years old…Replace all 4 shocks, leaking or not…You will not not believe the ride and handling improvement…

+1 to Caddyman’s post.

Would you just replace one shoe because it looked worn but the other one didn’t . Replace just one shock because of leaking it’s almost guaranteed the other side will start leaking in a couple weeks. Or at least that’s they way my luck would run.

+2 to Caddyman’s post.