check this out,look at the aluminum hub, look at the drum alignment tangs,and also they align the rims on the hubs also. after time they get worn,basically corode and fall to pieces,and dont hold the drum squarely on the hub,causing the vibration.also fare they oil packed hubs or the merritor sealed hubs?
see the wheel pilot diameter,and the drum pilot diameter,look at the tangs and see how badly the drum wobbles on them.
http://www.conmet.com/pdfs/Hub_Service_Manual.pdf
page 6 shows the tangs in better detail.also this is the manual of the hub.