I wouldn't take it with me personally, but here is a decent idea if you already have the spare transmission.

Get a large "Rubbermaid" plastic tub and pack the transmission in it. You can cushion it as appropriate, but I'd put it in a huge garbage bag (well sealed with tape!) and then spray Great Stuff (spray foam) in to fill the bin. Use "low expanding" foam, the kind for windows and doors.

Once dry, put the lid on the bin and seal it with a bunch of screws drilled right through the top of the bin.

Label it with your return address, and put a label on it where someone can write a shipping address on it.

Give a friend keys to your garage and leave the bin inside. If you run into a problem, call and instruct them to write an address on it and take it to Greyhound.

If your transmission blows up, get to a Podunk mechanic. It will take him a day to assess the situation, put your car on a hoist, and then pull your old tranny. By this time, the new one will be will on the way to him (if not arrived already) and he can pop it in and you can leave.

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
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'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
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'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
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