Most likely crud has fallen into the mechanism.  It isn't that hard to take out and clean up. 

There is a screw that holds the wood surround in place that is accessed from the transfer case shifter hole.  Then there is a spring clip that holds the shifter lever to the rod that goes to the transmission (under the truck).  Get new grommets for both ends of that rod, I assure you they are trashed.  

A2029920010 Collared bushing for shifter rod
A2109920010 Bushing on transmission end of gear selector rod
A0009944160 Clip for transmission rod ends

At that point you can just unbolt and take it apart for cleaning.

Hardly.  There are plastic bits and circuitry in there that might not take kindly to the aggressive solvents in Kroil or any un-stickum spray.  This calls for disassembly, cleaning and re-assembly with silicone paste. 

As long as the ignition is off, you won't get any new codes from the swap.  Most likely there are already codes there.  These shifters frequently cause silent faults of:

018 Selector Lever Improbable

114 Sporadic Selector Lever Improbable

The way the lever works is that only PRND is mechanically transmitted by the rod.  4321 is transmitted electrically with a digital pattern on several wires.  There are only so many patterns that are valid, the rest trigger this error.  When the contact plate gets dirty incorrect patterns get sent.  This usually clears up by itself as the contacts self clean certain extent as the shifter is moved.