Does your diff have a LSD fitted?
Should be an ID tag on it somewhere, either held captive by one of the bolts through the diff or stamped on (can't remember) and should give you a diff type. If it's 1041 or 1043 then it's LSD and needs an additive. If not, 75W80 or 75W90 will be fine and you can also use a product to quieten noisy diffs/gearboxes if it whines a bit.
Out of the two ATFs Volvo recommend for the box, i'd go for the Type G as it has enhanced friction modifiers over the "F" type.
As already mentioned thoguh, probably best to drain the gearbox completely and then refill with your chosen fluid.
