'89 chevy k1500 4WD suddenly not working

[tomeadie]

My 1989 Chevrolet half-ton K-series truck (5.7 engine, automatic tranny, 147,000 mi.) has four-wheel drive, which had always worked fine until about a month ago, when it suddenly just quit working. Now when I put into four-wheel drive, either high or low range, the front driveshaft spins, but the front axles do not engage, so neither front wheel turns. I assume this means the problem is somewhere in the front differential or axles or hubs, since the transfer case is evidently doing its job. So, how do I troubleshoot the problem? Somebody suggested to me that the thermal actuator is the likely culprit, but when I unscrewed it from the front differential housing and jumpered it to a source of 12 V DC, the plunger popped out about an inch or so just the way it's supposed to. I also checked the wiring of the circuit leading to the actuator, and it all appeared fine. The fuse for the 4WD circuit is okay, and I measured a solid 12.06 volts at the wiring harness that it plugs into with the ignition on and the shifter in 4H. So, I'm thinking that the thermal actuator thing is okay, although I'm not 100% sure. I'm worried that the problem may be an internal breakdown inside the differential, although I'm surprised that that would lead to a situation where neither front wheel works. I've known people with bad front differentials who got power to one front wheel in 4WD, but not the other. How exactly would I diagnose a problem with the differential? If there is a problem, what are my options? How hard is it to fix or rebuild it myself? What about buying a new, rebuilt, or junkyard differential? If the problem is not with the actuator, the wiring, or the internal gearing of the differential, what else might it be? Any advice would surely be appreciated.