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

In most locations, yes.

However, generally it must be proved that you intended to conceal (the usual word) the asset. Keeping it in your locked garage is usually not regarded as "concealment". Keeping it at your fourth cousin's garage out of state probably is.

It's extremely likely that any effective attempt to prevent your car from being repossessed means you lose the use of it anyway, so there's ultimately not much point in doing so, particularly since the lender is generally allowed to charge you for the cost of the repossession... you're just building up more debt. Even if what you're doing is not strictly speaking illegal, about the only logical reason to do it is if you know that you'll be able to pay off the debt in full in a short time and want to keep the car from being repossessed before you can do so.