I will bet your hubs a iffy. PVH trucks dislike the water. It ruins the seals that holds the vacuum in the knuckle. Which is how the hubs operate.
Are your 4wd lights flashing at all?
Easiest way to check is shift it in 4wd and listen for it to change, its not quiet. At least mine isnt. Try 4lo while your at it, thats even louder.
My suggestion is AVM manual hubs, even if your tcase motor is the problem. Its the best 100 bucks you can spen on your truck. Well if you dont like getting stuck that is.
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