The heads on the 4.0's are pure crap. If you do a cam and stage heads with a tune you will get around 40hp. The engine wakes up and does seem to be powerful. Downside is your looking at north of 3k for that 40 hp.
A 5.0 swap with a rebuild can be done for less. I got my donor for 600, and sold enough parts off make it free. So 800 ish for a rebuild 700 for headers add a cam and it should be a 75-100 hp upgrade. For less then 2000.
Granted it depends on what you can find, keep your eyes open If this economy hasn't screwed you over yet.
99 x-cab 4x4 4.0l auto
mods, cranked t-bars, silencer delete, k&n drop in, Rebuilt trans ,Trans Go stage 3 shift kit, 3200stall Fuddle Racing Torque Converter,
VANIR 63mm TB, Roush split rear exit exhuast, E-Fan, JBA Headers
Auto up/down both windows, OHC, EATC (soon to be modded)
03 Crown Victoria Police Interceptor