Idles rough when in gear

Is there an adjustment on the transmission? My son bought an Astro, I think it's mid 90s, and it acts like it's trying to pull too much when in gear and idling. It vibrates bad because of this. When in neutral it doesn't do it. Any help is appreciated.

After I think 94 there would be no adjustment to the trans. I'd take a look at the transmission mount if the vibrations only occur when in gear.