This can be the least expensive way to get five or six inches of lift. The project should come in between $15 and $600 depending on your vehicle and resourcefulness. Two cited benefits are more comfortable ride and a greatly increased articulation.
This project requires welding and if done improperly can be very dangerous.
Some companies such as Rubicon Express sell the necessary parts in a kit. These parts can be found cheaper individually. The following may be required:
Downsides to this are the vehicle will be easier to roll, you will probably have to address resultant axle wrap and, with another couple of inches from bigger tires you may not be able to get it into your garage!
Axle wrap has been solved with traction bars, RE's anti-wrap spring perch, and adding one rancho add-a-spring.
A shackle reversal is wasted on an SOA conversion.
Further reading:
Many companies market kits for the TJ. One inexpensive method is installing spacers above the coil springs at all four corners.
Other kits provide up to four inches by completely replacing the coil springs with longer ones. Just as with any other four-inch vehicle lift other work is required. Rubicon Express and Terra make kits that group members like.
Both Warn and ProComp sell kits to convert leaf sprung YJ & CJ Universal Jeeps to a TJ type coil suspension.
At this writing no one has reported on the ProComp conversion.
Early reports on the Warn kit are that it lives up to it's billing of greatly increased off-road prowess and improved street manners. Negatives are cost, with $5,000 being the most common quote received by newsgroup members, and increased body lean on the street.
An article on Dave Logan's Warn coil conversion and related links can be found here.
Concentrating on Warn Coil Conversions is a nice mailing list at Scott Tate's site. He also has some great pictures of a Coil-over YJ negotiating Golden Crack, Moab.
The new Edelbrock IAS shock has the best reputation in the group followed by the Rancho RS9000. The Rancho has five user adjustable settings and may therefore be better for special applications (buggy springs, SOA, etc.) The Edelbrock is self-adjusting and by group consensus gives the best highway ride. One side note is that the Edelbrock shocks cannot be used universally but must be specific for your vehicle.