When you reverse, then shift into drive, do you stop completely before you shift from reverse to drive? If you don't then that may have something to do with why your driveshaft broke. If you don't stop completely, your drivetrain is going one way when suddenly it gets power going the other way and this puts strain on the drivetrain. The driveshaft, being the weakest point in the drivetrain is usually the first (usually only) thing to break. Most of the broken driveshafts I've seen broke when going from reverse to forward. Another thing, was this the original driveshaft? The driveshaft may be a poor quality aftermarket piece which would also help to explain the premature failure of your driveshaft.
As far as replacing it, it's a very easy job. It shouldn't take you more than 30 minutes. Good luck!
