i need to figure out whats wrong with my truck anybody wanna help

i need to know why my truck wont shift when we hit 20 but it will if we go up hill or hauling things

2001 chevy truck. 4.3 v-6 some days it will start. Some days it won't. Turn key, all dash lights come on like it does when it does start. But nothing. Told maybe weak starter or bad positive cable or maybe neutral safety switch. Side post battery. Bad connection? Took cable ends off at battery and cleaned both battery and cable ends and put back on. ????