The trade-in value is now $31k, so I lose money on this idea but improve my net worth.
I still owe about $35.5 and have a 3.99% interest rate.
I see no way this improves your net worth. I mean, you said you lose money on the idea, where do you make it up? Maybe a table will help:
Net worth now: Net worth after:
'17 Tacoma + Value of New Car Itch Scratcher
-$35,500 loan - What you paid for New Car Itch Scratcher
mutual funds mutual funds
-35,500 loan
+31,000 from sale of Tacoma
See? The value of whatever you replace the Tacoma with adds to your net worth, but whatever you paid for it subtracts from your net worth. It's actually worse than this because you get nothing for the taxes you pay, and a lot of the transfer costs. Meanwhile, you have to cover the $35.5K loan with money from the sale of the Tacoma and from your mutual funds.
So, what are you really looking to do? It makes sense to me to look for something cheaper to operate (more economical), but that mostly moves you away from things that tow. For example, minivans, or small cars. It also makes sense to me to move to something older, so you don't have so much of a loan and depreciation on a "newer" truck. Do you run your own landscaping business, or are you just showing up and providing the labor? If you're just labor for someone else's landscape business, you don't need to accommodate them in your choice of vehicle.