Does the 1990 have a limited slip rear axle?
How do you know the 4WD is working?
If you don't have a limited slip rear end, one rear wheel and one rear wheel will spin freely, while the other 2 wheels don't turn (even though equal torque is applied to all the wheels). If you are on an even surface with uniform frictional characteristics, this is not a problem. But if you are on an uneven surface with highly variable frictional characteristics (like soft snow), then this can be a big problem. In this case you are reduced to a situation where the maximum torque you can apply to a wheel on an axle (either front or rear) is the amount applied to the wheel with the least traction on that axle. Because of the way soft snow packs in front of the wheels, it is pretty easy to build a barrier that makes the truck difficult to move. Since only one wheel on each axle is turning, it will melt the snow and make a super slick surface which means the total torque applied to the wheels is nearly zero, which won't move the truck against the walls built up in front of the none moving wheels. With a limited slip rear axle, both rear wheels are turning and can burn through the snow and get a gripe. Back in
1979, Ford also offered limited slip on front axles, so in theory you could have all four wheel turning. Later Fords don't offer limit slip front axles, only limited slip rear axles (and that was an option). It is possible that you have a limited slip rear axle, but the clutch backs are worn out. Pick-up trucks are often worse in snow that cars because they are so light in the rear end. And without a limited slip front axle I can see a case where both rear wheels and one front wheel are spinning without being able to move the truck because the rear wheels don't have enough weight on them to develop enough traction to overcome the snow bank in front of the front wheels (one of which is not spinning). Also, even though your 1990 might have good tires, are they wider than the tires on the 1979? My Father never had any trouble getting stuck with his old trucks, where as my newer trucks can get stuck on wet grass - mostly because the wider tires tend to float on the surface and not cut down into the mud or snow.My advice is that if you really want to go on snow - get chains.
Ed