Only have reverse in 2whl when in 4whl drive high. 4whl low works fine

I have a 98 gmc 3500 pickup with a 6.5 diesel automatic. Recently while plowing in 4 whl high I put it in reverse and a BANG (like when it slips out of gear) happened. Since then, when in 4whl high drive is fine but reverse only results in 2whl action. 4 whl low is working normally. Is this in the transfer case? Snow is in my forcast! Thanks. Dan