Noise

I have what sounds like a bearing noise coming from under the truck behind the front seats when driving it. Around 40 MPH is when it is worse. However after a few minutes of driving it goes away. I was told to check the center support bearing,they also said if it was a rear end noise it would not go away. It is constant whether cruising acceleraing or decelerating. It is an '88 3/4 ton 2wd with 454 and 14 bolt rear axle and two peice driveshaft. How does the driveshaft come apart? does this sound like a support bearing and how do I tell which one I have without pulling it apart, where would the part number be? Also I shook the driveshaft back and forth at the pinion, no play there, but when I shook it at the support bearing there was a very slight ticking sound, like metal on metal. Anyone help with some or all of these questions?