just bought a 1996 f250 the tranny slips in i believe 4th or 3rd gear but it shifts extrememly hard 1-2 2-3 4-5 the previouse owner said it happens every couple months the light will flash on the OD and he dumped a small bottle of stuff in it thats technically an addivite for the rearend(cant rememeber the name) and it would normally go away. i dumped the little bottle in and after a few days it quite but only for about a week. he never worked the truck and i use it to haul large loads of firewood. it has 236k on the stock tranny. and ideas or is it just time to rebuild it?