Watch the Ear Training online guitar lesson by Karan Andrea from Guitar Player Wanted: Vocals A Plus
Here is something that you can do in addition to the vocal exercises. Ear Training will do at least two things for you. Since your first idea of singing pitch comes from your ear and your brain, the more fine-tuned your ear and brain are to pitch, the more accurate your vocal pitch will be. Second, you may be so tuned into the sound of your guitar that you don't really hear your own voice. If that's the case you may have trouble hearing when your pitch is off or you may just have trouble focusing on hearing your own voice regardless of pitch.
Sit down with your guitar and play a simple scale or set of notes. You can plan an arpeggio, major scale, pentatonic scale, a melody, a riff - whatever you want to play. Then sing it back to yourself. When you sing it back, listen to what you're singing this will start to connect your ear to your voice. I suggest you also record yourself so that you begin to get an idea of what you sound like. I know it might be a little scary at first but you will get used to it, you're just not used to hear your voice outside of your head.
Because I sang long before I played guitar, I have the opposite problem. Occasionally when I'm playing and singing I don't hear my guitar at all which is pretty unnerving because I'm not sure if I'm playing in tune and in time. It doesn't happen very often but just be aware that now that you are splitting your focus, and your brain may not immediately take to it without some retraining.
Once you start being able to hear your voice sing along with the notes you are playing like George Benson, you don't have to do it in performance like he does, but try it when you practice.