John,
Ok, you are going to get many different opinions on this issue. Here is mine... I would have used 75/90 Synthetic gear lube for both applications. Your diff may have already had synthetic gear oil, If so, you would be good for a lot of miles. The hubs will be good with synthetic, by just checking the level.
So, the answer to your question is NO, you did not do anything wrong. What you did is better than conventional oil in both the diff and hubs, However, in our MH applications, lighter weight synthetic oil is much better than conventional oil.