Probably need to go through a process of elimination. Squeeks can come from just about anywhere under these old trucks. Try to see if it coincides with the rotation of the tire or drive shaft if possible. Causes can range from the above mentioned areas to loose tire/rim lugs, axle bearings and even a cracked drum.
Here's some previous posts on the driveshaft u-joints, hanger/carrier bearing.
http://67-72chevytrucks.com/vboard/s...d.php?t=416247
http://67-72chevytrucks.com/vboard/s...d.php?t=294596