Yeah, adumb is right. I don't know your stats, but I don't know a whole lot of guys in the 19-20% bf area that can do several good sets of pullups. Also, work on your core! Almost all resistance exercises are assisted by your core but pullups are highly assisted by your core. About a quarter of the battle is stabalizing yourself and it takes a solid core to do it. You should also take the advice that rip gives in his last post. Also if you want to get stronger, no matter if it's in your upper body or lower body, or lose weight you should do compound movements, dips, rows, bench presses, squats, deadlifts, and eventually pullups. You can do curls and trunk twisters all day, but people who get strong and big do compound movements.
Also, a bit of advice on form. With back exercises, you really have to get the angle of pull correctly depending on which muscle group your're trying to hit. It's very easy to place too much emphasis on your traps when doing back exercises, and it can be only because of a slight miss on your grip, where your elbows are, or where you're pulling the weight to on your body. A gereral rule is, the farther your elbows are from your body, the more emphasis you put on your traps and rhomboids. Try it, stick your elbows straight out to the sides and act like your doing a pullup-feel it in your upper back? now try it, but stick your elbows almost straight out in front of you. notice the difference? Try moving your hands a bit closer on the bar, just a bit, an inch or two, bring your elbows in a bit, and when you pull up, arch your back tilt your head back a bit, and act like your gonna touch your nipples to the bar. Also don't let your elbows fly out like chicken wings. If your're using pullups as a lat exercise, this is how you should do pullups, and as you get stronger, you keep the same form but start to move your hands back out on the bar. This is why people who know do their bent over rows with an underhand grip and keep their backs as parrallel with the floor as possible. Overhand grip at a 45 degree angle works your traps and shoulders, not your lats. Anyway, this variant position on your pullups will really target those lats. It will rectuit a tiny bit of help from your biceps and make the exercise a bit easier, but the isolation this variant produces far outweighs the tiny cheat of the biceps and is not only THE best lat Exercise IMO but the Pullup is essential if you want to be strong and get bigger. It is the KING of the upper body compound movements. More so than bench presses. Especially when you can start adding weight. Remember your back should be as strong if not stronger than your chest. That is, you should be able to pull more than you can push. Humans are apes, and we got our start in the trees. This is why, from an evolutionary standpoint, backs have the potential to be much stronger chests.