I wouldn't go out and replace a hard drive unless I knew the problem was the hard drive. Your problem sounds like it's a hard drive problem but it could be something else.
Today's logic boards aren't exactly "tough." The electrical traces on the logic board can crack easily yielding really erratic performance, and a lot of the internal cables aren't cables at all, but rather mylar strips with conductive copper traces etched onto them. Any time intermittent contact exists somewhere in the system it can cause the system to lock up erratically or behave oddly. For example, what happened to the cables connecting the logic board to the display, the wireless connections, and the camera, all of which are located in the lid of your laptop when you "caught" the unit as it fell. Were the jolts enough to cause problems with them?
Todays HDs are a lot tougher than they used to be. If impact wasn't actually made I'd be surprised if the disk was the source of the problem. I use a product called Scannerz with FSE to test my drives but they make a cheaper version of it called Scannerz Lite that can test your drive before going out and spending money on a problem that might not be a problem.
Personally, if I were you I'd take your unit into someone familiar with them that's willing to give you a free evaluation before spending money on anything, including Scannerz or Scannerz Lite. Granted, their excellent tools and it might very well be the HD, but common sense tells me making such an assumption may be unwarranted without adequate evaluation. For example, some time ago I had an old PowerBook 12" model and anytime I travelled the Airport card would dislodge slightly and manage to lock up the system during a boot...and that was just from vibration related to traveling. In that case the solution was always simple: reseat the Airport card, which back in those days was accessible. If vibrations from an airplane trip could cause an Airport card to unseat, what make you think that a sudden grab of a falling unit couldn't do likewise with yours?
Translation: get it checked out, dude!