Is it against the law to hide your car from being repossessed in tx?

Yes. It's a violation of Texas Penal Code section 32.33 (b) if a person "destroys, removes, conceals, encumbers, or otherwise harms or reduces the value of the property" involved in a secured debt, which would include the car in a car loan.

There's a very small loophole in that the person doing so has to have "intent to hinder enforcement of the interest or lien", but this is mostly closed by the provision that you're assumed to have had such intent if you've missed a payment and you failed to immediately hand over the car when the lender asked for it. If they haven't actually asked you to return the vehicle, you might be able to avoid the criminal charge if for example you can prove that you normally leave your car in a friend's garage or whatever, and that it wasn't there out of any desire on your part to conceal its location from the lender.

The seriousness of the crime depends on the value of the car. For most cars it will either be a state jail felony ($1,500 or more, but less than $20,000) or a third degree felony ($20,000 or more, but less than $100,000).