Sandcrawler, real or fake?

Both. A Miniature for wide shots of the moving sandcrawler, and a partially built full size version for the close up shots with people:

From page 18 of The Best of The Lucasfilm Archives:

Off-the-shelf parts are an important part of big-movie model making. The sandcrawler craft used by the Jawas to scour the flatlands for minerals and plunder, utilized four store-bought panzer tank tracks at its four corners. The final sandcrawler model was a fully motorized, radio-controlled vehicle three feet long and constructed of birch plywood covered with plastic and detailing. It was built with such powerful motors that its radio operators could make the sandcrawler pop a wheelie, although when the cameras rolled the miniature vehicle only had to lumber over the Tatooine wasteland.

Please note:

The following images are from page 60 of Star Wars Chronicles:

Only the lower part of the full-scale sandcrawler was actually constructed for filming. Matte paintings were used to complete the vehicle, and a radio-controlled model was used for shooting in the California desert. The full-scale sandcrawler was two stories high and 27 meters long. The completed model measured 56 x 125 x 66 cm.