I've got an 89 D21 Hardbody V6 4x4. On mornings after sitting in the
cold all night (during winter) I often hear a grinding noise coming
from the front hubs (I  think) once I start driving. When I come to a
stop, the grinding slows down and makes kind of a thud sound. Usually
after this intially grinding/thud, the sound does not come back.

My theory is that because of the cold, the gear oil in the transfer
case is so thick that even though 4x4 is not engaged, the front axles
are somehow partially engaged?? but the the hubs are not? The only
solution I have found to the problem is to engage 4x4 right from the
getgo and run it like this for 10 minutes or so,  to warm things up,
then disengage. This isn't ideal because I don't like to run 4x4 when
the streets are dry, or at high speeds etc. Any thoughts out there?
Does it sound like my hubs are wearing out and need replaced or maybe
just greased? Should I switch to a lighter weight gear oil in the
winter? Would switching to the manual locking type hubs be a better
option?

Thanks,

jon