BIG vibration and walking all over the road!

[75-K5]

Got the truck back from the exhaust shop yesterday and timed the engine, so now it runs pretty good (except the vacuum advance diaphragm is busted). Now it's time to concentrate on other things. When I'm going between about 25 and 35, the whole truck shimmies like it's going to pull itself apart. It used to do it even worse, but replacing the carrier bearing helped a little. Could it be the driveshaft out of balance? How much does it normally cost to have a 2 piece shaft overhauled?

When I'm going this speed, I also hear an extremely loud rattling coming from what sounds like right outside the firewall on the drivers side. Its so loud and fast it almost sounds like a ratcheting, much like the noise you hear when you raise a chain hoist.

The truck is also walking all over the road. If I hold the wheel straight, the front tires will spontaneously shift to the right or left a little while I'm driving. It's nothing too scary, it only varies my path by a few feet either way, but it's definitely enough to notice. I have about 1/4 turn of play in the steering wheel, is this the problem or is it alignment? Could it be related to the rattling?

Sorry about all the wacky questions and thanks for any answers!