To use a 4wd trans you will need to tear the trans down completely and swap output shafts.
My 99 lost reverse. I also had no engine braking in first. Which points to a broken one way sprag clutch. I drove about 10k with no reverse, it takes a bit of planning when it comes to parking.
Another possibility is a problem with the low reverse servo. Typically a gasket failure.
I got a trans off eelbay which was a POS with a crap warranty. If you get a trans to swap in make sure it has a good warranty.
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