Connection of vacuum for sinski big boy 150

https://www.youtube.com/watch?v=dZh3Iu4LYa4
If the truck has a vacuum hose to the transmission (to the vacuum modulator), the line will be a metal tube attached to the large tube that the transmission dipstick is in. It will come up and connect to a hose that will run to the base of the carburetor.
The function of this line and the modulator it goes to, is to give the transmission some sense of how much throttle is being applied during a shift to make the shifts softer or harder depending on load. The transmission will work without this line, but the shifts will be hard as if the throttle was all the way to the floor.
If the hose is broken or missing, there will also be a vacuum leak at the carburetor, causing a rough/high idle and possible an intermittent backfire.
If the truck is new enough, the transmission will be electronic and not have a vacuum modulator at all, and will have a plug with several wires connected to it on the drivers side.

If you're talking about an automatic transmission I suspect you might have a vacuum leak you can spray soapy water all over the hoses and see if bubbles are produced top part of the engine intake area of the carburetor helped create the vacuum if there's not enough vacuum it would have trouble shifting otherwise you might have actual transmission damage

possibally the purge canister

Should only be vacuum on 1 of the 2 hoses to the IWE, the other is a vent line that is just hanging in the engine compartment.

Did someone redo the vacuum lines and accidentally connect the vacuum system to the vent on the IWE ? If so, this would cause some problems with the 4WD system.

Remove the vacuum line running from your carburetor to the distributor.. Plug the nipple on the carburetor so you don't lose vacuum.

try the local parts store, give them the information about the truck and they can assist you with the proper plugs and vacuum hoses, make sure you know what hoses you need.

It sounds to me like there is still a pretty good vacuum leak somewhere, and you should also check for any dis-connected or missing vacuum lines on the intake manifold, at the carburetor, and the large vacuum hose to the brake booster unit and the vacuum valve at the brake booster unit for any vacuum leaks or broken vacuum lines/hoses, and they can be hard to find because cracks in the vacuum hoses like to hide on the under-side of the vacuum lines/hoses especially the hard plastic ones. Also the carburetor could be loose or the carburetor base gasket could be damaged, the EGR valve gasket could be damaged, or the intake manifold gasket could be damaged, and these are all the most likely places to suspect a major vacuum leak.

make sure the vac line is connected to the intake if so replace one way valve at booster

if u have a V8 test the upper intake manifold for a big vacuum leak at the base where it bolts to lower intake, use a can of carburetor spray, if it is leaking you will hear it when the carb spray hits the leak.

sounds like it might be a vacuum problem check all the vacuum hoses and spray some wd40 around the base of the carburetor while idling. if the engine revs up then you have a carburetor base gasket problem or a leaking vacuum hose