Trans problems for a dudge durango rear weel drive ?

ok now i have a dudge durango my trans is not shifting it only go up to 24mph and dont shift to drive theres no leaks or the fluwid is not black or burned or there no smoke so what can it be it can drive but will not shift to secound gear on get up to 25 mph and every thing else works it reverse fine only thing is that the speed sencer when out but it still was driveing tho be for i had the problem >> ???? please can come one help me THNAK YOU MY FRIEND