Reving or Idiling noise when driving

Hi, I have a 1998, Chevy, Blazer, 4x2, 4.3 engine, with 125, 000 miles on it. The engine is original. I just got a tune-up because of this idiling or reving noise that I have been hearing for the last month. It sounds like the engine won't go into gear after accelerating from a stationary position. It almost sounds like if I was driving a stick shift that sound it makes if you don't switch from 1 to 2. Once the car gets up to about 30-35, I hear what sounds like it hasn't switched gears or something and it is just reving. I also, about 2 months ago, had a fuel pump installed and I didn't hear that problem before or after it was installed. I got the tune-up and the sound hasn't changed. I didn't get a major tune-up, they changed the spark plugs, the wires, the air filter but not the oil filter. Please tell me what could it be because, I don't want my engine to go out on me. Could it be the oil? Or the belts? Or their are too many miles on it.