John wilkinson

I am pretty good at diagnosis on trucks but this stumped me. I have a 2015 Sierra 3500 cab chassis with 6.0 gas, manual transfer case plow and landscaping truck. My issue is the truck will not go into 4 hi or low when the plow is mounted, it switches into low and hi without the plow assembled. Nothing is leaking at all, fluid is correct level, actuator works (with plow off), front shaft is in good condition, cv shafts are newer, nothing but the differential dog bone arm bushings is bad. My question is will those being worn give the axle assembly just enough twist to not allow engagement or is it something deeper. No noises come from front end that are rotational based, as best as I can hear while driving. Thanks for any input.