Typical immigration wait times are so-so around 4pm at TBIT: the average is often around 20 minutes (depends if you're a US citizen or not), but the max wait if you're in the back of the line can be closer to an hour. And it doesn't help that Terminal 8 is about as far as it gets at LAX: there's a shuttle bus outside that runs every 10 minutes or it's about 6/10ths of a mile walk.
It should be doable if all goes well (since the airline sells it as a single ticket, they at least consider it possible): your inbound flight isn't delayed, no extra-long line at immigration or customs, no extra-long line at TSA security. But it doesn't leave a ton of margin if there are delays. Since this is a single ticket, the airline will rebook you on a later flight if you don’t make it.
The catch is, it looks like that evening flight to Austin is the last United direct flight there of the night (there's a Southwest flight at 7:20), so if you miss it, you could well be stuck in Los Angeles overnight, as there really isn't an alternate routing that makes sense to get you there faster. Whether that's a risk you're comfortable assuming will depend on your circumstances and your need to be in Austin on time.