hard shifting.

I have a 95 lumina ls with 150,000 KMS on it and in the past few months it has started to shift hard from first to second. I was told that they used vacume modulators in these things until 97 and that it might be the vacume line or something. It also shifts harder when im going slow then if im accelerating faster. If it is infact the vacume line, what can i do about it? Where is it located exactly? im assuming the large block under the engine is the tranny...thanks