* Engine: A six-cylinder engine weighed less than a V8.
* Options: Power steering, air conditioning, added features all increased the weight.
* Equipment: A truck with a camper shell, toolbox, or other added equipment would weigh more.
* Fuel level: A full tank of gas adds considerable weight.
To find a close estimate, you'd need to know the specific engine and options of the particular truck. You could try looking up the specifications for a 1988 Dodge D100 in a vehicle specification guide (sometimes available online or in libraries) and it might give a range of weights, or at least a weight for a base model. Even then, it will only be an approximation.
