Replace both. I had a VERY noticeable clank/clunk if I hit a bump in the road in my ST for along time. I literally hunted the clank/clunk down for 5 months. Everything I read and watched on Youtube told me it was the front end. I damn near replaced the whole front end and it didn't go away. Strut bearings, tie end links, hardware, bushings, ect. I almost bought coil overs out of desperation.
One night it rained, and I decided I was just going to go enjoy my ST in the rain in a big parking lot. I beat on it pretty good and went home. Next day I went to work, and went I started the ST it made the same exact clank/clunk without moving. Instantly I knew it was a motor mount. Ended up being the top passenger side motor mount. It literally drove me crazy for 5 months. The rubber had crack and leaked all the dampening fluid out, and it was flat. It took me beating the snot out of it for the motor mounts to finally collapse and give up the goat.
