That can get a little bit tricky to tackle if you don't have any experience with it. Any chance you have a buddy that's down for the cause and also happens to be an electrician? If not, any chance you could bring your grow indoors and get an electrician in there to run proper wires to it, proper breaker and fuse box, and good grounded outlets? Are these the only plants you have going out there? If so, they could easily be kept under a couple of CFL's for a few days while you have someone come in. It's not going to be all that inexpensive to bring an electrician, I know... maybe you can find a good electrician who would be willing to barter something in exchange for his/her work? If you decide to go the electrician route, tell them you plan on running some heavy duty shop equipment in there and they will wire accordingly.
If all of that's not a possibility, I would go the extension cord route.
Sorry bro, didn't mean to put "the fear" in you. I would bypass the plugs in the shed until you can sort out the grounding issue. I don't think a trip to Walmart tonight is necessary... but I would go first thing tomorrow. For tonight though, just put your fingers up against all of your cords, plugs, and outlets to make sure that they don't feel warm.
its the first thing i do when i setup a new grow room... new braker and heavy duty wire....
