Blue Water Bridge, Windsor Detroit Tunnel, Ambassador Bridge are prime targets, considering Dearborn is 20 minutes away. Any of these are prime since you would disrupt commerce from two countries.
I think bridges would be a disruption, but both Charleston and JAX have big military installations and a successful attack on either of them could be a more serious problem.
Charleston, of course, also has a military brig where some of this slime is twiddling its thumbs and waiting to be sprung. Not going to happen, IMHO, but that doesn’t mean they aren’t going to try.
The tunnel is a different story. I think a blast in the tunnel, unless somehow "directed" towards the ceiling would end up being more like a pipe bomb, with the blast dissipated out either side. Anyone inside the tunnel at the time of the blast would be in serious trouble but I don't know if the tunnel itself would be permanently out of action. The tunnel runs under the riverbed and any blast would have to burst through several feet of material before the river itself breached the tube. Before 9/11 they would fill the tunnel with cars. After 9/11 they only allow enough traffic in on either side to fill the customs plazas, which are outside. Under these circumstances it would certainly be a suicide mission and I don't think it would be very successful.
In any case, semis carrying freight do not use the tunnel as a rule. It is mainly utilized by passenger vehicles.