So when I turn on my wipers while the windshield is dry, they stop about half way down! They don't go all the way down when I turn it off. I have to get out the car and put them down with my hand.

While off, I move the arms and up and down and they easily move up and down the window! They go up about 3-5 inches.

When it's raining and the windshield is wet, the left wiper blade slams on the side of the window while going up, then when they go down, they almost slam the plastic.

Do I need a new motor?
or new wiper arms?

 

Check to see if the wiper arms are torqued to spec

 

this and align them when they are on their off position before tightening.

 

How can I check that? I unbolted the arms and moved them up a little, that helped the slamming a little when going down but when I turn the wipers off, they still stop in the middle of the windshield!

 

I moved them up, they seem aligned!

 

this is a common problem. replace your wiper linkage if aligning doesn't fix the problem.