tranny wont shift out of first gear
 

hey guys , ive got a 2000 blazer 4x4 4.3 4dr. auto . it has a noticable leak under carriage of tranny - i felt my tranny taking a bit longer to shift to second gear so i stopped at parts house checked fluid , it was way low .. i bought a 24oz Lucas trans stop leak and a qt of fluid . got level right. for the first mile it was fine.. then and now it will not shift out of first gear . just trying to get an idea of what im up against , if i could fix it myself or go the smart route and have it diagnosed and fixed w/warranty .. $$ is my issue right now ..Thanx for any help you may be able to give