why wont my 95 bronco engage in 4wd

There should be an electronic solenoid on the side of your transfer case , sometimes you may be able to tap on it and get to engage, or it may be no good. That's where I would start.

check to see if your hubs are locking properly.

my bronco has a new coil,distributer,and module and will start up and run but sometimes it won't spark its just sometime it will and somtimes it wont what does that mean