I'm not totally sure the vibration is related to the driveshaft. I'm sorta thinking out loud here, not being argumentative. I appreciate the input. I don't see any vibration in the center of the shaft anymore, and what I do see seems to be coming from the drum end of the axle. I could actually see the drum and the trailing arm (which was supported on a jack stand) hopping up and down about 1/4" at certain speeds. I realize it's all connected and vibration could be trasfered througout the whole system, but the drum hops like a tire out of balance, although not as violently. Does this still sound driveshaft related?
I started a separate thread about the heater core. I just took some pics I'm about to post. I'm absolutely positive it's the correct core. Outlets just aren't quite in the right locations. Here's a link :
http://67-72chevytrucks.com/vboard/s...d.php?t=746310