The Nissan Path

I have a 2001 pathfinder LE 4x4. It will not down shift when needed like when getting on the freeway it will not downshift into lower gear to gain speed. When you put your foot into it, it just bogs down and even in the neutral the rpms do not rise. It keeps getting worse and worse. Getting more frequent even at lower speeds. I've always kept up on fluid changes so I really don't know what the issue is and i thought is try this before getting bent over at a dealership. Please HELP!!