Blazer Vortec issue

Im new here and hopefully can find a bit of guidance. MY 95 Blazer Vortec is at 116,000 miles and has an overdrive problem. Runs decent in low gears but when taken out on the highway, doesn't want to shift into high gear. Also, it seems to be less powerful when climbing hills with passengers in it. Any advice will help.

Try changing the fuel filter

when it goes up hills do u make it down shift?
and what tranny is it 700r4 or a 4L60
and check the fluid level