OTL they missed him by a decade. Shaka was assassinated in 1828; the Trekkers came up against his much less competent successor, Dingane.
Even against Dingane, the Trekkers lost the first couple of battles. Isolated Boer units were cut off and wiped out at the Weenen Massacre, and then the first Boer attempt to penetrate Zululand in force was blocked and turned back at Italeni. However, the Boers made up for it all at Blood River, where they occupied a fortified position, built a laager out of wagons, and mowed down wave after wave of attacking Zulus until the impis finally wavered and broke.
Blood River cost Dingane his throne and his life; Zulu power went into eclipse for a generation, and though it would rise again in the 1860s, never again would the Zulus be a serious strategic rival to the Cape Colony.
Okay, so [handwave] let's give Shaka another dozen years of life. Say his mother doesn't die, so he doesn't go into the murderous depression that OTL led to his assassination. A more long-lived Shaka may have all sorts of knock-on effects, of course -- but let's focus on just one: how would Shaka have dealt with the threat of the Trekkers?
It's hard to imagine him doing worse than Dingane. Dingane's treachery with the Retief party accomplished little but to enrage the Boers. The Weenen attacks killed a fair number of the invaders, but were not followed up, allowing the Boers time to recover and summon reinforcements. And the Blood River battle was an unmitigated disaster. It didn't help that the Zulus under Dingane were divided, with a fair number following Dingane's mild-mannered brother Mpande. Mpande stayed neutral for much of the conflict, keeping thousands of potential fighters off the field, and then eventually allied with the victorious Boers. It's impossible to imagine anything like this happening under Shaka.
If Shaka is still alive, I see two possibilities. One is that a united Zulu nation manages to see off the Trekkers. I actually think this is the more likely outcome. Yes, the Boers had muskets and horses, and the Zulus did not. But the Zulus were disciplined fighters -- at the command level, considerably more disciplined than the endlessly quarrelsome Boers -- they outnumbered the Boers almost 50 to 1, and they were fighting on their home ground. If Shaka were alive, I suspect that Zulu advantages in numbers and discipline would be pressed relentlessly until the Trekkers gave up and went away. (Which would have been entirely possible -- the other branches of the Great Trek were busy opening up and settling vast lands to the north and northwest.) In this TL, Zululand stays Zulu, at least for a while, and the tiny settlement at Durban probably gets wiped out as well. There's no short-lived Boer Republic of Natal, and the Cape Colony does not expand to the east for at least another generation.
The alternate possibility is that Shaka ends up killing off the entire Zulu nation in a fruitless attempt to stop the Trek. I think this is unlikely, because I think Shaka could have beaten the Boers -- but if he couldn't, I think he'd keep trying until every Zulu impi had been decimated. In OTL the Zulu state was badly battered, but it survived and was able to attempt a strategic comeback. TTL, I think it might get smashed out so badly as to disintegrate, losing all political identity. (And possibly ethnic identity as well -- the Zulu "nation" was still quite young in the 1830s. It had been forged out of a cluster of related tribes under Dingiswayo just a generation or two earlier. OTL the process of amalgamating everyone into a "Zulu" identity wasn't really complete until the 1850s.)
Either way, we get a South Africa that looks quite dramatically different from OTL.
Thoughts?
Doug M.