If, as you say, the actuator itself is all right, but it drops out of 4WD when the engine is loaded, look for a vacuum problem in the lines or in the reservoir. The vacuum reservoir, located behind the right front bumper, can crack or leak, especially if there's ever been an impact there. The feet can break off and leave an inconspicuous hole. Also check the vacuum line that runs to the reservoir, underneath the battery. Corrosion can eat it. If you also notice that the heater switches to defrost under the same kind of conditions that cause your 4WD failure, this is a sure sign of a leak in the shared vacuum system. If you don't hit pay dirt there, check all the 4WD-specific lines, including the rubber connector to the actuator, for damage and looseness, and look for rust pinholes in the metal lines as well. There are a couple of low spots in the metal lines that can collect moisture. I had this problem, and it just about drove me crazy until I found the tiny pinhole in one of those lines. A vacuum pump and gauge can be really handy here. Finally, check the vacuum switch on the transfer case. These can sometimes go bad, either leaking or sticking. Fortunately, although the actuator on the axle is expensive, the switch is pretty cheap - last time I bought one it was about 12 bucks.
I had an 87 for a few years, and every single failure that can happen to this system occurred at least once, except that I never broke an actuator shift fork.
