This is the transfer box otherwise known as angle gear or bevel gear
You can see on the left hand side to the right of those two ridges there is a part number.
The bit at top centre is the output shaft which bolts to the propshaft.
You can see the bolts on the right hand side which is where it bolts to your gearbox.
You can also see the collar/sleeve on the right hand side which joins the gearbox to the transfer box.
The picture below shows the input shaft of the transfer box. You can just see the edge of the splines on the picture. If these are rounded, you need a new transfer box. If buying second hand, make sure these splines are good.
The picture below shows the collar/sleeve/coupling. These can be purchased new from Volvo for approx. £60. There was a revision to these which makes them stronger so I would suggest fitting a new sleeve even if you fit a second hand transfer box that comes with a decent sleeve.
These transfer boxes are more compatible than people think. The output shaft can be unbolted from your old and new transfer box so that it mates up to the propshaft regardless. The input shaft must have the same amount of splines on both boxes though.