Short Cut through South Carolina & Georgia
Ken, I have travelled (by van) south from the junction in VA of I-81 and I-77 many times, and here is my favorite route to FL. It's a mix of Interstate and secondary highways and I think it's very efficient.
I-77 south from I-81 into and through NC and to Columbia SC. Get off I-77, exiting to the right, near Columbia SC at exit 1, then keeping left to turn left onto US-321 southbound. 321 is mostly one lane each way through about a dozen towns with mainly Scandinavian names, then it joins US-301 in Ulmer. When 321 veers off to the south, stay on 301 all the way to and through Georgia until you get to Folkeston GA, then watch carefully for GA-121 veering off to the right. Take GA-121 to the Florida border, where the road becomes FL-121. CAUTION: slow to 45 as you cross the bridge into Florida, and smile pretty at the State Trooper on the left side of the road, then carry on south into Macclenny (and then continue on FL-121 as above). Drawback of this route: only one rest area, just a mile into GA, and it closes at about 5:30 PM.
Hope you like it!