Vibration

A week or so ago my '85 SR5 ExtraCab pickup developed a vibration. The vibration was there at about 55-60 mph. When going slower or faster it was as smooth as ever. Sunday I took a 100 mile trip and by the time I got back home the vibration was there from about 20-70 mph and when I slowed to turn onto my street the vibration had worsened to a definite knock and shook the whole truck. This morning I got under it and I could see that there was about 1/8 inch side play in the rear u-joint so I decided that since one had gone bad the other two were probably about ready to be replaced too so I replaced all three of them. This took about 99% of the vibration away but there is still a little when I let off the gas and coast about 55 mph down to about 50. Does anybody have any ideas what could be still causing the vibration? I took the center carrier (with bearing) to a mechanic and he said that it looked okay to him but when I put it back into the truck I could tell that there was a little bit of side play in it; not enough to see but I could feel that it was there. If it is measurable at all I would guess that it couldn't be more than

1/32nd in side play. Could this be a cause for vibration? The bearing rolls smoothly and the rubber in the carrier doesn't have any cracks and nothing appears to be broken. If the problem isn't in the carrier where else should I be looking. As I said earlier, I replaced all three u-joints and I hope that it could be something else rather than the carrier. I called around and the price of a new carrier is from about $125 as Schucks to $285 at the local Toyota dealer. I don't want to replace the carrier if I don't have to. Any suggestions??? jj