For the tolls, I guess besides the older idea of toll truck lanes, maybe some gaps of I-81 could have some ETL(Express toll lanes) or HOT (High Occupency toll) lanes similar to the concepts used for I-495/Capital Beltway around Arlington and I-95 in Maryland.
I digged the archives of misc.transport.road and I founded that old post from 2004 about tolls on I-81.
https://groups.google.com/forum/#!topic/misc.transport.road/XvlxtWmxBm4%5B1-25%5D
I think the idea of truck-only toll lanes (and trucks prohibited on car lanes) is dead and is not coming back.
HOV/Toll lanes probably do not make sense here, because there are no large employment centers, little or no transit or other bus service (IIRC Megabus has one or two trips each day between Knoxville, Tennessee and Washington, D.C. with a stop in Christiansburg (a "suburb" of Blacksburg), and the
Virginia Breeze bus service is run by a sister company of Megabus between Blacksburg, Christiansburg, Lexington, Staunton, Harrisonburg, Front Royal, Dulles Airport, Arlington and Washington, D.C.) and those employment centers that do exist are spread-out up and down the I-81 corridor in Virginia.
If there is a desire to go with tolling, then I think the tolls should apply to all traffic using I-81, though if there are gantries between the interchanges (like MD-200), then perhaps the first 10 to 20 miles could be free of toll for drivers of cars with E-ZPass transponders. Note that I strongly suspect that elected officials in the corridor would not like this idea at all, though they might like it better if it was presented as this or do nothing.