Late to the party but in one of my more formative years of playing; as wise and very skilled guitar player told me; it's different when you play with a drummer. Even if you've practiced with a record/tape (hey, it was the 80's and very few folks owned a CD player when this was said!)

I'm just adding a wee bit of context to the volumonous (and awesome) stuff Chris posted. Still, practice and play along with the songs. Pick songs strategically. If AC/DC, don't practice 'Walk All Over You' (for instance) or 'Whole Lotta Rosie'. Fun to play but these are a bit fast but can lend itself to being a little sloppy and still be ok. Play 'TNT', 'Back in Black', 'Dirty Deeds' or 'You Shook Me All Night Long'. Not that there aren't a million from AC/DC but these are all good rhythmic songs.

What they all have in common is the ability to tap your foot along while you play. It's likely you will just tap you foot without really knowing/trying.

Good rhythmic playing is also about groove. Your riffs/notes/chords being played fall in to the rhythmic pattern of the song. Some times it's a little slack (falls after the note a little bit) or on top of the beat. Each gives a different vibe.

This is why playing along gives you a sense of note placement. Just keep on practicing until you hear that the riff you are playing is locked in to the recording. It takes time to get timing. Even if you have a good sense of rhythm, it takes time.

Example; The first song I learned was Rush' 'Fly by Night'. I practiced it from memory (I didn't actually own it on album at that point). So I played it and it sounded ok for a beginner. Then years later, I decided to see if I remembers all the parts I learned and played along to a friends CD. I remembered the parts but my timing was garbage. At that point, I was a pretty solid player and had good timing...except on that songs. Because I didn't bother actually learning the riff/note placement.

A little more food for thought.