That price is for a complete 2WD to 4X4 conversion with solid axle.

The axle alone will set you back $2000 for a good Dana unit in working order. Then you have to get brakes working with the factory items. Then new wheels, which won't match the rear, so you need new hubs with the same bolt pattern instead. Then getting the ABS working, or discarded. Then steering components need to be modified or replaced.
Then you get the fun part of finding a transfer case.

That also includes labor and design, which would easily be 50+ hours at $50/hour.

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Regards,
Ian