it wont go into 4wheel drive its an automatic shifter in the floor

there may be a hole, or crack in the vacuum system, in my Jimmy (nearly identical vehicles), i had to get out and manually close the valve, after shifting inside the cab. it would then stay in 4-wheel, until i disengaged it (in the cab) if you can locate the piston, give it a try. for the jimmy, it was just to the passenger side of the engine, behind the battery. Start your engine, engage 4-wheel, hop out, then squeeze that piston closed - if it stays closed, then your problem will be the same as mine. if so, get these hoses fixed as soon as possible - when one hose starts wearing out, the others are usually right behind it. this can mean future serious problems, if your bronco is an automatic.