What could cause a rough hesitation when shifting while on the interstate?

I assume you are talking about an automatic trans? If so, it doesn't sound good. Have you serviced trans lately? Sometimes brand new fluid can cause old trans to slip. Auto trans are complicated animals. Don't take it to one of those AAmco type places unless you want to spend a small fortune. The only trustworthy place I know is David Colbath Trans but that is in San Antonio, Texas. Ask your regular mechanic first, someone you trust. Good Luck! I had 2 identical Geos with the same problem. Turned out to be weak fuel pumps. Check your fuel pressure