1989 Chevy Astro low speed vibration

I have an old 1989 astro van, it works great on the highway, and any speed over 50km/h, but between 20km/h and 50km/h it shakes the whole van, you don't feel it in the steering wheel, and if you let off between those speeds, it lessens alot, under load it will near shake you out of the seat. Driveline seems tight. I just bought it though and the bellhousing bolts were loose, I thought that might be the problem so I tightened them all up and it is still the same, I thought maybe someone had the trans. out and it had problems so they put it back in but you can tell by the undercoating it has not been out. Hope you can help, thanks.