I have the front and rear axle assemblies from an 89 tracker which have the 5.125 ratio. Can these be installed in the 87 Zuke straight axles that I just bought???

Nope.  The front can go into the front of the Sammy, but the rear gear set is a bit larger.  You need to get a Hybrid axle from either Hawk or Trail Tough.

One problem with just swapping the front diff though is the aluminum housing.  A lot of people have broken those off playing on the rocks.