94 k1500 350 vibration

[dmkilgo]

I just bought a 94 k1500 with a rebuilt 350 in it, and I'm trying to figure out why I'm getting some vibration around 55-60 mph. After playing with it some, I've noticed that the vibration stops as soon as I take my foot off the gas pedal (but I'm still going the same speed), so I'm thinking it must be something more related to the RPM at 55-60 mph in overdrive than the speed. Since it stops when I take my foot off the gas, I'm assuming that it's not the tires or driveline, because they are still spinning at the same rate. Could it be the engine itself? If so, what do I do to correct it? Anyone have any ideas? Thanks.