You are right, the axle shafts from either a 16v or 2.6 would work, but I don't think ones from a W124 will work, it's a wider track. Parts numbers are not the same. 2.6s come up in the yards near me once in a while, but be aware that they are not easy to pull. I've never completely separated one from the rear diff but removing the rear diff along with them is a task in and of itself. You'll need a long breaker bar and a 30MM 12 point socket plus a hex bit socket, regular sockets, and wrenches to remove the diff from the rear subframe and from the driveshaft.

Otherwise, you can buy used on eBay or new at about $600 a side, this is the parts number: 2013508110

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.