No 3rd gear HELP!!!

Hey guys I've got a 94 wrangler w/ a 4.0 and auto trans. Ever since my mother-in-law ran the trans about 3 quarts low it has been hesitant going into 3rd gear. Now it won't shift into 3rd at all. 2nd, 1sr, and rev are fine but no third. Do I need a rebuild, or is there something I can check. Kinda at a loss, as transmissions aren't really my thing
thanks in advance