Emissions and hoses

That looks typical for a vacuum power brake booster. There should be a round check valve plugged into the booster, and the right end of the hose plugs onto the port on that check valve. Look on the back of the throttle body or carburetor for the threaded fitting, or possibly on the back of the intake manifold. There should also be a vacuum hose routing diagram specific to the optional equipment on your truck, under the hood or right above the radiator.

There is over a dozen vacuum diagrams for your truck. These four are for a 2.8L with manual transmission. The first two are Federal and the last two are for California emissions.

If you can hear the vacuum leak, can you follow the sound to the port and find it that way? At least you would find where one end goes. The only other hose I am familiar with that is that big in diameter is for the PCV valve.

Also consider visiting a pick-your-own-parts salvage yard to look at a similar truck.