^^^^what Sam said. Also your 86-87 spindles take the same bearing for inner and outer. 88-89's take a small bearing on the outside and a bigger inner one. 88-89 spindles are the same....it is the hubs that are different with that spacer in there on the 89. Someone correct me if I'm wrong.
the spindles are not the same. 88 has the step (to act as outer bearing support, no spacer between bearings), 89 has a taper (and requires a spacer between the inner and outer bearing). They are only interchangeable if you pull one of the bearings out to add or remove the spacer depending on whats going on.
I maybe overstating the obvious, but to me pulling the bearing out was a big deal so I bought like 3 sets of hubs before I figured this out and got what I needed for my 88.