Just changed my diff fluids out about a week ago - 45k miles on the fluid (factory fill). I sent a sample off for analysis and it came back squeaky clean. My vehicle seens mostly highway miles and has not been through much if any deep water.

As with everything oil related, your specific usage is going to set the timing for when to replace fluids. Unless you know the previous owner of your truck, it's safer to assume that no fluid maintenance was performed if there are no records of it.

If you've driven through deep water without having completed the diff breather mod you may want to change the fluid just in case regardless.