You are correct that as of 2018, like-kind exchanges can only be done for property.
If your business vehicle was fully depreciated, and you traded it in (didn't sell it), enter your Sales Price as $0.
TurboTax will ask the FMV of the vehicle when you first started using it for Business, then ask Basis for Gain/Loss and AMT Gain/Loss, which you can leave blank.
If you used Standard Mileage, TurboTax asks Depreciation Equivalent (not more than FMV used previously). If you used Actual Expenses, you can enter Accumulated Depreciation from 2017 Form 4562.
That's all you need to enter; TurboTax will report your Gain/Loss. If you used Standard Mileage, you will have no Gain/Loss (screenshot).
Set up your New Vehicle with a Cost Basis of what you paid for it.
An article on Business Use of Vehicles:
https://turbotax.intuit.com/tax-tips/small-business-taxes/business-use-of-vehicles/L6hi0zzzh
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