eh i wouldnt worry about it.. alot of trucks around here see speeds in excess of 60mph in 4wd.. and your birfs arent packed anymore lol.. anybody who actually uses their toyota axle in 4wd knows all about axle seals.. they are done as soon as that shaft turns. so you already have gear oil in the knuckles and have oil soup 
but eh... dont worry bout it none
if you shim the knuckles right they last a long time
