Nothing.
Great truck "BUT" I purchased it new in August 2002. It is a 4x2 Regular cab XLT with 2.3L 4 cyl. It is a great truck and other than a Rhino spray in bed liner I have done nothing to it. The only problem I have is the rear likes to dance on rough roads, this is normal due to the lightness of the rear and it being sprung for a heavy load. The only thing I wished Ford had done was to make the limited slip rear available with the 2.3L engine. I live in Minnesota and this would be a great help in the winter snows. Other than this I believe it is a well designed and put together truck and I will be owning it for many years to come.
I am also puzzled as to the unavailability of the limited-slip differential on the four-cylinder Ranger. Chevy offers it on the four-cylinder S-10 and Colorado--why can't Ford?
I ended up equipping my 2002 2WD Ranger with a 3.0 V6 to get the limited-slip. Combined with the anti-lock brakes and the second-gear start feature in the automatic transmission, it's really easy to drive in the winter here in Michigan. It's never gotten stuck once. I love it!
It does bounce an awful lot. The supercabs are even worse. Wonder if a heavier toneau cover would help.
I bought a 2007 4x2 with the 4cyl engine and was also surprised by the "jumping around" rear end and the lack of the limited slip option too.
I ended up buying a better limited slip rear differential (a Detroit Truetrac) than the stock one (the truetrac has no clutches to replace) and it cost less to buy and install that the stock one would have.
The rear ends jump around because they are both so light and heavily sprung for carry loads. When I added a new fiberglass canopy the extra 100 or so pounds really smoothed out the ride (and keep my gear and dog dry too!). Try throwing some weight in the back and you'll notice a huge improvement int he ride.
On a similar note, stock Bronco II rear swaybars fit. I picked up one from an 1988 from a junkyard for $30 and now it corners like it's on rails!