I have a fence around my back yard; the posts are just set into the ground and tamped firm (no cement). I plan on putting my old jeep under the deck for winter storage. To do this, I will take 2 fence panels off and wiggle the post betweem them until I get it out (not a problem if I do it when the ground is moist). I think this would work for you - especially if this is for long term storage rather than as a garage. It depends on the amount of times you'd like to access this area.

Another option would be to install a removable post in either concrete or with bracketry to a firm base. Be creative.