It sounds like the OP has a typical one car garage, w/o much excess height. He probably won't have room for rolling an unused car 12' in the air from 4-post to pallet rack storage.
As was mentioned, all-thread vertically, with horizontal unistrut members, and some plywood as the platform surface should suffice. May I suggest some steel L-angles, like Simpson products, already drilled? Bolt them to rafters, double nut w/fender washers the all-thread, and the same on the bottom end. Bolt the wood to the unistrut. Depending on the size you want, you could use more unistrut at right angles to the bottom ones, to provide middle of the load support, or just use more unistrut on more vertical all-thread, w/the wood bolted-down. You could also use wood 2 x 4's between the unistrut, then the deck surface of plywood. With a bit of time for layout, drawing up a materials list, getting the stuff, and doing the install, it could be a full day. I think I would do the installation another day, that way you would be fresh when you tackle it.