will not engage in 4x4
Check your 4WD actuator on the front Axle housing…
With the limited info you gave and to get you started…
the first thing would be to check the fuses and relay for the 4x4 system. And you need to check the vacuum lines for leaks and for damaged or cut wires.
a common problem is a Faulty actuator: The actuator is responsible for engaging and disengaging the 4WD system. or you can have a problem in your transfer case, but I would look at that last and try the easy stuff first.
Troubleshooting Dodge Ram 4WD: Why It’s Not Engaging (truckguider.com)
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No experience with this particular 4WD configuration, but for 2008 Dodge truck, I expect the 4WD hub actuators are located on each of the front hubs. They are probably powered by the force available from engine vacuum. They require the full force of the vacuum to work. Vacuum leaks are a common reason for them to stop working by reports here in vehicles so equipped. Suggest to use the forum search feature to see what others have said, link upper right this page.
My Ford 4WD truck uses manual hub actuators. That means I have to get out of the truck and rotate a little plastic knob to get them into the 4WD position which allows the front driveshaft to power the front wheels. Yours is a more convenient configuration. You don’t have to get out of your truck. But there’s a price to pay for the convenience. .
Leave the monster truck jumps to Grave Digger. 
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Or BIGFOOT.
What??? In the south that is part of the fun of owning an older 4X4 truck, jump that thing, heck, I’ve cleared RxR tracks in cars, I told this before, but my wife blew out my R/R strut jumping RxR tracks in my car once, took me years before she admitted it… lol
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