Will not shift into 4wheel drive

This would be the shift linkage bushing, very common. When switching through the gears, the gear selector lights in the dash probably don't move from P.

Is your car truck an all wheel drive.. if so thats what it does... it senses the terrain and shifts accordingly! If not all wheel drive you have a problem! post the info on the vehicle! Brand Model and year! Maybe we can help!

Does it shift to 4wd high range ? If so then shift to 2wd . What is going on here is the transfer case motor is stopping due to the motor position sensor has gone bad which means that you replace the transfer case motor assembly. But if you wish to make it a 2wd for awhile then remove motor and manually turn shaft . This can be done by raising and supporting rear axle truck in park remove motor turn shaft slowly with pliers and rotating the play in drive shaft. There are stamped markings on back of case showing gear positions..

You can "shift on the fly" at up to 45mph - just let off the gas and pull the shifter back to 4hi. DO NOT try going to 4lo while moving.

Another way is to stop, put it in neutral, and pull the shifter back to 4hi or 4lo.

This is vacuum activated - so if you can't get it to engage - the vacuum motor is failed - or you have a disconnected or rotted vacuum line.

Cherokees have the command - track. You'll see a lever next to the transmission shifter that has letters 2H, 4H, N, 4L on it.
You can shift from 2H (2wheel drive high range) to 4H (4wheel drive high range) at any legal speed.
If it's an automatic, I recommend just taking your feet off of both pedals, so there's not a lot of torque being passed thru the transfer case.
If it's a standard I recommend stepping on the clutch when you shift, but it's not necessary.
To go to Neutral just pull the shifter back one more position to the N position. To shift into 4L (4wheel drive lo range) you must be stopped.
I also recommend shifting the transmission into neutral when you do this. Push the shifter to the right, then pull it back.

Thanks!

4 wheel neutral vehicle will not move that for towing or moving around --put in 2wh --4hi or if have 4low transfer case has to be in a setting to move

does it have auto hubs, if so check the vacuum switch at tranny side

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It is the shift motor for the 4wheel drive - only problem is that a shop has to program the new shift motor to the blazer or it will keep doing the same thing

try turning the steering at a complete stop fullly left and right. If the steering is resistnt then you are in 4 wheel drive. vehicles in 4whl drive don't like to steer unless they are moving.

this is common on these trucks just put your witch in 2 wheel and go ahead and drive it will let loos when it sees the correct secquence and drive a little and shift to netural.
just neve do it from high to low range