On RWD cars like my bmw 318i you can swap out the whole differential in about 2 hours. My car left the factory with an idiotic 4.11 limited slip differential. I picked up a 3.64 differential from the JY for $50 and I couldn't be happier. Whats more, I sold the 4.10 for $250. Not only was the 4.10 diff annoying on the highway, but driving around town I found myself using 4th gear around 25mph and 5th over 30. When Accelerating from a stop light I would have to shift to 2nd before I even made it across the intersection. Really really annoying.
The beauty of these old cars is you can find about 10 differential ratios in the JY between 2.79 all the way up to 4.45