I think you've probably found the culprit - the axle is probably knocking. It is difficult to say how stiff an axle shaft should feel; the best thing to do is to remove the axle from the vehicle and check the torsional wear at several points along its "in and out" position. There are a few videos of this procedure on youtube such as this one =>

How to Test a Mercedes Rear Axle for Excessive Wear - Kent Bergsma - YouTube

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!