I recently added 80/90 to my rear end in my 88 240 Wagon 5 speed. (took almost a quart-leaky pinion seal!) Now (and maybe this had nothing to do with adding oil) in the morning, when I start down my long driveway, the rear end snatches and clangs (my neighbor asked what the noise was). The noise then disappears completely as I head out for the day. Does not repeat even after sitting for a few hours, but will in the morning after sitting all night). I visually checked my u joints,my flange bolts are tight...could I have a posi rear...there is no tag...would the symptoms be like that if I did and failed to put additive in it? Can I get additive at a parts store, or have to use Volvo's? Help!! Anybody ever had a noise like this?? It sounds like spider gears (in my mind, at least).

These differentials are notoriously rugged. It's unlikely that you damaged it and even more unlikely that you did something by adding oil.

To check if it's an LSD, jack up the rear end. With the car in gear (std) or in park (a/t) and the e-brake off, turn one rear wheel. If you can turn it (and the other wheel turns in the other direction), then it's an ordinary differential. If you cannot turn it, then it's an LSD.

Limited slip differentials are not common on a 240.

But if you DO have one, I believe that it requires an additive -- certainly worth asking about.

A failing rear axle bearing (aka wheel bearing) can make a snapping, crunching sound. The last few failures I had fooled me into thinking I had a pebble stuck between pad and rotor.
--
Don Foster (near Cape Cod, MA)

The 240 rear end clang-snatch noise could possibly be comming from the normal wear and tear(stress) that the rear end bushings receives on a daily basics. A 200 series rear bushing kit could be the answer to your problem. Jack the car up with the rear wheels off the ground with the car safely supported with jack stands, now you can inspect the rubber bushings.
Good Luck