On a 1998 GMC jimmy 4x4 What would keep the front differential from locking in?
On the transfer case, there is a set of vacuum lines. If any of them get old / brittle or melt, it will cause a vacuum leak. These trucks have a vacuum actuator under the battery that locks the front axle. if this unit is getting no vacuum, leaks, or is not connected to the cable, the problem you describe will happen. You can test your front axle by first putting your jimmy (or blazer) in 4LO, then shut off engine / apply parking brake. Jack the front wheels (both) off the ground at the same time. Have a friend try to rotate one wheel at a time while you manually push the vacuum diaphragm in on the unit beneath the battery. If the wheels stop turning, that means that your diff is ok. You need to start checking vacuum lines. if the wheels keep turning, well that's another story. Sometimes it takes a few rotations and a lot of fiddling to get it to actuate, cause normally you are driving forward slowly when it engages. You have to "replicate" that.