'89 Cavvy RS 2.0L vibrating?

I figure it's something with the timing or something of the like... but when I hit a certain RPM as I'm driving, the whole car vibrates and it can be felt pretty easily all over. I tested it by slowly accelerating it as I'm driving. I don't have a tach or else I'd be able to tell you exactly what RPM it's at, but... about in the middle of second gear @ normal acceleration it vibrates (around 36mph) and then it shifts up into third, and gets about halfway into third, and vibrates again (about 47mph). But, if I go 2mph above or below those speeds, then it's totally fine. It also vibrates when I take my foot off the gas, and goes down to the same RPM again. Any ideas?