James Duff apparently still hasn't seen the light with TTB suspensions. So they vehemently discourage you from using non-TTB parts on them.
That's unfortunate too because there was a fellow on here a few years back that worked for Duff who tried to do some things with the TTB (researching it for a new kit I assume), but he failed to heed much of the advice that was given to him about setting up the steering properly.
If you haven't read already, the steering has
BY FAR the greatest potential to make or break a TTB project with lighter-rate springs. If the angle of the linkage isn't correct (in particular, the angle of the tie-rods relative to the axle beams), you're likely to encounter issues with the suspension jacking and squatting during acceleration and braking due to bumpsteer (not to mention the atrocious handling and squealing tires).
This issue is also far more common than many people realize.
http://www.therangerstation.com/Magazine/winter2008/steering_tech.htm
http://www.therangerstation.com/forums/showthread.php?t=499
http://www.therangerstation.com/forums/showthread.php?t=29
http://www.therangerstation.com/Magazine/Fall2003/MaximumFlexForTheTTB.htm
As for EB coils, yes they do work (quite well actually), but as Bobby mentioned, some mods to the coil mounts are needed.
I would suggest purchasing
Duff's dual shock coil towers as this should make installing the upper end of the coils a bolt-in affair (the dual shocks will also offer better control of the body roll inherent with lighter-rate springs). You still will need to mod the lower mounts on the axle though (and depending what radius arms you have, adding another lower mount for the 2nd shock).
Another user here modified a set of
Duff EB lower coil mounts with good success (thread
here) though unfortunately all of the pics got lost when Photobucket pulled their BS some months ago. There still is some good reading in the thread text though).
Since you have a reg cab, Jeep Cherokee XJ coils are another option you might look into. They are a little lighter-rate than EB coils (230PPI vs 300 or so), so they work well on non-Supercab trucks. I run Skyjacker 6" XJ coils on my BII for maybe a bit over 5" lift. On a Reg cab Ranger I'd expect to see around 4.5" out of them (which should work well with a set of 4" axle brackets)
IMO, linear rate coils are best for TTB suspensions (variable or progressive rate coils tend to restrict your uptravel during articulation more than is necessary).