1995 Chevy Suburban not getting power

Im having problems gettin power at high speeds.I took a trip through the mountains and it started fine, but then trying to climb, it was to a slow crawl.I prob could have gotten out and walked faster than it was driving. Passing on the highway, I feel like I cant overtake the car im passing.I can hear my truck sputtering sometimes while my foot is on the brake and its idling. Seems to me something is preventing me from just getting up and putting my foot in the gas, its to a slow crawl. Fluids are good. But I admit I havnt had a tuneup in about 3 or 4 yrs, but I dont believe the tuneup is preventing me from gettin my power