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on your Jimmy you need to put it in 4 hi and up under the battery tray is the actuator for the front dif. make sure the vac hose going up to it has vacuum. if it doesn't then start tracking the hoses going down to the transfer case. one goes to the actuator, one to the vents on the transfer case and transmission and one goes back up to the vacuum canister, it's located on the hood on the drivers side. if the canister isn't getting vacuum then nothing is going to work. i just had to repair the hoses in the truck i put the trans in. the 4 wheel drive now works in it. i would have never know this, the hoses drop over by the exhaust and get hot and burnt and start cracking and breaking up and leaking. this can also cause some minor running problems. also. one last note. where the hoses attach on the transfer case. that connector has to go on correctly. it has a pin hole in it to line up on. if you put in on wrong things won't work.
Try listening to the transfer case try to engage.The vacuum actuater on the firewall above the distributor was not allowing vacuum to the front axle actator. You can put the truck on jackstands (all 4 wheels) and check to see if the front drive shaft engages. If so, then it's probably the vacuum actuator or the axle actuator.
Sounds like your front axle is trying to engage but does not engage fully.check your vacuum going to the t-case switch and then if that is ok check your vacuum at the front axle actuator.It is located under the battery tray.
In addition to transfer case engagement via an actuator on the "T" case there is a front axle actuator. On my Blazer which I'd bet is very similar.. There was a vacuum diaphragm under the battery which operated a cable to "lock in" front axle. make sure you have vacuum supply when asking for 4 wheel operation and make sure cable is not seized. If this checks out o.k. back to transfer case. your question asked about front wheels locking so I assumed your "T"case was working
that vacume line goes back to the transfer case, the end that goes behind the motor hooks to a steel tube so it does not get hot from the exhaust., my guess it should be hooked up so it will feed vacume to the vacume ball so it can lock in the axels. if you start the car and shift into 4 wheel drive and listen under hood you should hear a hissing sound, that is vacume, if you can't hear it there, you may have to craw under the right side to hear it, probably came unhooked underneith the car
those are engaged by vacuum on the pass. side on the fender well under the hood there is a diaphragm that will pull in when put in 4x4 many times the vacuum lines get old and crack or get pulled off. The lines go to the transfer case and to the vacuum canister as well as the diaphragm on the fender well. If you have a vacuum leak it will cause it not to pull so if your vents don't change over good you have a leak elsewhere and that needs to be fixed. I don't like the vacc. engaged 4x4 but when it works it is a light weight shift on the fly design that you don't have to worry about the shift motors getting stuck or corroded.
Does it sound like a loud winding sound coming from the front right side? I have a 2001 Jimmy and the 4wd tries to engage when driving up steep hills and going at highway speeds. The actuator vacuum switch engaged when the transfer case vacuum switch sends the vacuumed air to the actuator. What is happening is their is either a leak in the vacuum lines, a faulty actuator switch, or a faulty transfer case vacuum switch. When the jimmy has enough air pulling in, the extra vacuum is trying to push the actuator switch in when it is not supposed to. It is a much cheaper fix than a transmission. GMC should have recalled the whole 4wd system on any of the S10's, the JImmy's, and Blazers.