When you say rebuilt - did the block come out? If so, was the block deck checked for flatness/milled and was the new head also checked for flatness/milled. Did you have teh head magnafluxed prior to installation? If the head and block were good or cut & the head was fluxed and passed, the only other reason I can figure a leak is, the gasked was cracked? It was running OK for the first week and then all of a sudden it started ******* on you?
Not that it matters for your present question - but standard or auto trans, 2WD or 4X4? Also may want to add your location, goes by your join date. Sometimes this helps determine things based on weather. Good luck and keep us posted on wht you find.