shudders at 70

[acl]

going to and fro the midwest truck meet in kc this weekend and my truck had a shudder that started at 55 and got worse the faster i went. Had wheels and tires balanced and helped a bunch but still have a shudder that starts at 70. not as bad as before but still there. I have fairly new drums on the back, can those be balanced? anyone heard of that? I am sure they are not all perfect when they cast them, just did not know if that can be checked. I plan on having driveshaft balanced (anyone know where this can happen in kansas or kc mo.) also might put a diff. set of wheels and tires on just to see if they are not right yet. Am i missing any other possibility that you have had?It is not real bad thru the steering wheel so i don't think it is up front. Thanks for the help. Gary