Ok i have a 98 blazer with 154k it shifts fine like butter super smooth but after like an hour of driving from 1st to 2nd it shifts a little harder like u can feel it shift like if the pressure went up all other gears work fine.... this is weird why is this?

 

Here's one of the results from a search for "hard+shift"

Everyone having 1st to 2nd tranny slams HERE!

As Kyle said in response to the original poster, make sure you diagnose before you just throw a valve body at it.