probably a vacuum system leak. Older dodge trucks had the front transaxle engage by a vacuum line. Make sure your pump is working, and that there are no leaks. Then it should engage
Dodge trucks are safe to drive when driven properly. If you are using them for four-wheeling, the safety tests will not apply. Vehicular deaths are declining, and safety features have been installed on Dodge Trucks: Air brakes, Air bags, tire monitoring and adjustable top belts. They often receive a 5/5 rating.
Dodge Ram two wheel drive trucks camber corrections are made using the slotted pivot bar on the upper control arms. On the four wheel drive trucks the camber is preset at the factory.
I've got a 1985 Dodge D150 2wd and there is a detailed vacuum hose diagram on a sticker on the inside of the hood.
It is on the top of the transfer case.
Check for vacuum leaks.
I would start by checking vacuum lines
D100 are 2 wheel drive trucks. W100 are 4 wheel drive I own a 1975 dodge D100 and it is a 4 wheel drive.
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Out of the known car companies, Dodge was the first to start 4 wheel drive in 1934 with their trucks.
Check for vacuum leaks,
On a Dodge Ram, it runs from the intake manifold, to the switch on the transfer case, to the front axle shift actuator.