Welded differential
A differential whose spider gears have been welded solid, so both rear wheels turn at the same speed permanently. $100-300 at a shop.
The cheapest way to make a rear-wheel-drive car drift, and it genuinely works. The differential comes out, a shop welds the spider gears, and it goes back in. Published figures put this at $100 to $300.
Why it is necessary
A factory open or viscous differential sends power to whichever rear wheel has least grip, which in a drift is the unloaded inside wheel. So the inside wheel spins, the outside does nothing useful, and the car will not hold a line no matter what else you have bought. Getting both rear wheels turning together is the single modification that turns a rear-wheel-drive car into a drift car.
What it is good at
Completely predictable, because it is always 100% locked - no ramp angles, no preload, no transition to learn. Nothing left inside to wear out. And it costs about a quarter of a used limited-slip differential, which on a first build is money that becomes rear tires and event entries.
What it costs you
On the street it chirps and skips the inside tire in car parks and tight turns, it wears street rear tires faster, and it is harder work in the wet because there is no slip at all. It is also permanent - that differential is now a welded differential forever, which matters if you might sell the car.
Mechanically it is harder on the axles, the driveshaft and the differential mount than a limited-slip differential, because every shock load goes straight through with no compliance. At stock power that is rarely a problem. Once you add real power it is the reason the drivetrain reinforcement item exists.
Before you weld
Fresh differential oil, and check the differential mount. A locked diff loads that mount in a way the factory never intended, and on several chassis - the S-chassis in particular - the mount area is also a known rust spot, so the two problems compound.
What welding actually involves
The differential comes out of the car, the carrier is opened, and the spider gears are welded to the carrier so the two axles can no longer turn independently. It is a half-day job for a shop and it is not reversible - you replace the differential if you change your mind.
Ask whether the shop has done one before. A badly welded diff can break, and a broken diff in a car with no compliance in the drivetrain takes other parts with it.
Questions
Can you daily drive a welded diff?
Plenty of people do. It chirps and skips the inside tire at parking speeds, wears street tires faster and is more work in the wet. None of that is dangerous, it is just worth knowing before rather than after.
Can a welded diff be reversed?
Not meaningfully. You replace the differential. Factor that in if the car might be sold.