Check the tree for caterpillars, there is a moth that lays eggs on apple trees here that eat the stems of the apples which cause them to drop, you may have something similar. If you do find an insect culprit, do not use a pesticide. Come back here and we can work something out.
Also, do you fertilize your tree? If so with what and how often?
Furthermore as we try to figure out your problem here I would advise you to read up as much as you can on Avacadoes both on the net and at your local library. Try searches like: Avacadoes and fruit set, Avacadoes and fruit drop, Avacadoes and fruit, Avacodoes and fertilizers and so on.
Also, do your research with a critical eye. If you read on something that advises you to use synthetic fertilzers, read it and understand, bu, realize the synthetics end up causing more problems than they solve.
I hope this helps,
good luck with diagnosing the problem.