The 1988 K2500 does not use a vacuum modulator. You only have a throttle valve cable (TV cable) wich is located on the left side of the transmission. the cable goes to the throttle linkage. If incorrectly adjusted or broken the transmission wont shift up/ skid.
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This year of GM Transmission doesn't have a vacuum modulator, it is all electronically controlled. And yes a shift solenoid can cause this but that should turn the check engine light on and set a fault code.
Automatic or standard transmision. If it is a automatic check the dip stick and see if the oil is red or black, and if it smells burnt or not. If it is black and smells burnt, you need to overhaul your transmission, which is not unlikell for a 4x4. If it is a std you are still on the verge of needing a new clutch.
Will the engine rev up in park ? I would assume the fuel pressure is low because of the lines being connected wrong, but that would not be easy to do. The pressure line should be bigger than the return line.
Drop the pan, odds are the shift solenoid is out, its located at the front of the trans on the driver side. I'm sure your check engine light is on that will comment about the timing/shift solenoid being bad. I had the same problem with mine. The solenoid had plastic coming out of the inside. To remove it you'll find a small C clip holding it in, just take a small screw driver and pull the clip out. The part cost me like $36 from my local dealership. And its a longer solenoid. I believe its the 3-2 downshift solenoid. I do have the part number if you need it.
See if the tranny has an exterior modulator valve. If so remove the vacuum line and see if there is tranny fluid in the line. It will be located on the pasenger side rear of the tranny. What's happening is the engine is sucking tranny fluid through the vacuum line directly into the intake and it doesn't burn well.