2002 Chevy Impala transmission / shifting?

[jbizzle]

My impala has been shifting hard into first gear. When I'm at a stop it feels like I'm getting rear ended and nobody is behind me. My father knows cars very well and said there is nothing wrong with the transmission and that it has more than enough transmission fluid. I'm not quite sure why it is shifting so hard if it's not the transmission. I know it needs a new resignator but that's the exhaust, it shouldn't effect the shifting. Any suggestions? PS: I wanna keep low budget on fixing it