I have a '96 C1500 p/u, approx 175k miles on it. V6 engine in it.
Almost a year ago, I was driving to work. About 4 miles into my drive, I enter the highway. Upon trying to accelerate to highway speeds, the engine suddenly bogs down and I am unable to accelerate to anything above 40MPH. Later that day, while on my lunch break, I tried driving it again. This time, breaking 15 was a challenge. By the time I got out of work, while the truck would idle fine, as soon as I would touch the gas the engine would bog down and stall.
Due to lack of funds, the truck sat for a couple of months. I could still move it around, as long as I didn't touch the gas at all. Finally, around October I got it towed to my friendly mechanic. This is where the fun really starts.
I suggested a massive vacuum leak to him, because of how sudden it was and I thought on one occasion I heard a hissing sound. I may be mistaken, though. He put a fuel pressure gauge on it, said the pressure was low, and $400 later there is a new fuel pump installed, beautiful fuel pressure, but still the exact same symptoms.
He then claims to check "everything" and says he found no compression on all 6 cylinders. This sounds very fishy to me, as just 2 years earlier I had done a compression test on it myself, and while I can not remember the exact numbers, I remember all cylinders were close and within spec of a brand new engine.
Anyways, He then got it in his head that ALL the valves are burnt up, and it will be cheaper to replace the engine than do all that work. At that point I asked him to tow the truck back to my house.
Now, here I am, nearly a year after the problem started, about to try and tackle this myself. From what highly limited knowledge I have, I can tell the engine is starving for something. What that something is, is my problem. I think vacuum leak is still on the table, I am going to make sure the fuel pressure actually is good. I can take out the o2 sensors and see if its a clogged cat. Other than these things, what would you suggest? Are these even possible? I have very limited knowledge, currently do not own a code reader (it somehow was left inside a car I sold 4 years back, and I haven't convinced myself I need a new one yet), but I am eager to learn, and would love to get my hands dirty and get the old girl running like new again.
I may have forgotten some detail you may need, and due to my work schedule getting information posted same day may be a challenge, but if you have any questions I will get you the information as soon as I can.

 

Put a vacuum gauge on it and have a helper crank over the engine.