1999 Blazer tranny problems
 

I have a '99 Blazer and i've not had problems with my trannys so im new to tranny trouble. Ok when I get into a little she doesnt seem to want to go anywhere. The RPMs shoot up but the speedometer barely moves. I've has an import mechanic say just get new plugs im sure thats it. Then had a car ethusist say the "shift sensor" in that year and simular years go bad. And im thinking its the entire tranny cause i've heard they are just bad. Can anyone please help. If you need more details i'd be glad to answer, anything to save money and pinpoint it on my own. THANKS ALOT GUYS!