I would want to listen carefully to the sound and verify whether it is related strictly to road speed (in which case a tire, wheel bearing, etc. might be the cause). If related to the gear selected and/or the engine speed you'd have firmer evidence that it is the trans (or maybe the engine). I guess the transfer case is after the trans so if the noise is absolutely related with road speed you'd still have that to consider. Does the noise change on acceleration vs coast, if you put some extra load on the car (by riding the brake pedal for a few seconds) or when you turn left or right?

My tranny does that when it is cold, but goes away after about 300 yards and a shift to 2nd.
Definitly flush the fluid. Also, flush the bevel gear box by pumping out the old junk and pumping new. You will be surprised how bad the old fluid smells.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

no, not at all normal.

Have you verified the ATF fluid level? The very first thing I would do is follow the directions on Bay13 and do a tranny flush. Most likely nobody has ever changed the fluid in this car. These models are very hard on tranny fluid and it goes from its new pink/red color to dark brown in about 30K miles. At your mileage it is probably like hershey's syrup.