1995 gmc jimmy...won't go into 4 wheel easily...took me about 45 minutes today to get it to engage, going back and forth and it seemed to engage and then "pop" out. finally engaged and stayed engaged....could this be the vacuum lines? and...where are they?

Its a vacuum opertaed system that pulls on a cable that is located under the battery tray the part is called an actuator it is likely that this part is leaking engine vacuum and dosn't generate the force to pull the cable enough to fully engage the 4 wheel system. Or loolk for a vacuum leak at the transfer case or the switch at the same place. You can also disconnect the cable from under the battery tray and pull on it to see if you can make it shift into 4 wheel and hold in that pisition whole you test drive it if it stays the actuator is the problem and the vacuum decay is pulling the cable back and disengageing the 4 wheel drive

There are vacuum lines at the rear of the engine that t-off into different directions. These can become deteriorated with age and get mushy and collapse. The main culprit is underneath the battery tray. You'll have to remove the battery and the tray to get t it. The hose is connected to a vacuum actuator that controls the front axle engagement. It is always coming loose because of the sharp bend in the hose where it hooks up to the actuator. There is also a hose that runs down to the transfer case. While you're under there checking those vacuum hoses, go ahead and check the electrical connections at he front axle and the transfer case.