Hey Darren,
My "secret" to moist and tender butts or shoulders is to rub the meat down with a fairly heavy coating of rub (all over), then roast the meat between 225 and 275 for the first 4 hours, and then wrap it in two layers of "good/thick" aluminium foil with a good dousing of your finishing vinegar based sauce (at least a cup)before you wrap it up. Put it back on the heat (225-275) for another 4 hours and then pull it off the heat. Let sit for at least 30 minutes (allows the moisture to be drawn back into the meat as it cools), and then open the foil. The meat should practically fall apart in your hands, yet not be mushy.
I use a stick-burner smoker with an off-set firebox so there is absolutely no direct heat source. Not sure about your charcoal grill if it is direct or indirect, but my experience is that direct tends to dry out the meat more. I specify 225-275 as I am still having a rather difficult time keeping my heat steady, so it varies throughout the cooking process on my smoker.
Try it and GOOD LUCK!!! Let me know how it turns out.
Tom (aka "Another_Q_Lover" & "BBQ.TOM")
CMJ w/ KCBS
CBJ w/ MBN, NCPC, NCBS, & NCBBQA
OBJ w/ BBQ Critic.com
CTC w/ KCBS
Pitmaster and Chef - 4 Lil' Pigs Competition Barbecue Team
Author of "A 'Q' Review"