if it´s real cold the gear oil in the front axle gets so "thick" wich makes it hard to engage 4wd but my guess is a vacume leak. 4wd is partially controlled with vacume and the front axle angege system is known to be problemmatic.. It´s the thing thats is on the passenger site of the front axle in the back.. there are 2 vacume lines and an electric plug.. disconect all three remove the 4 bolts and clean it up. thats where an air compressor is handy.. Next step check out all the lines envolved in the vacume system the main hose from the vac pump to the brace cylinder is known to soften up. my advice is remove it and go to an hydrolic store and ask for another that is vac proof.. in this hose ther is a T that goes conects to the vac lines on the fire wall from there to the right it leads to the crusecontrole unit. To the left it goes to the plowermotor and the transfer case. the line to the transcase runs frum the fire wall down and behind the 6th valvecover. in the back there it goes from a hard plastic line to a idiodic rubber knee and goes from there to the top of the transcase. sorry man hard to explain from memory just check it all...
Ram 2500 12v 5.9 cummins turbo, 47RH Auto. all stock. exhaust, transmission and boost gauges. home made manual lock up controlers fuel plate a bit frw. starwheel detuned.. 38" superswampers custom fender extensions.