bronco 4 x 4 question

My 91 full size bronco acts funny sometimes in 4 wheel drive. If I have it on in snow or dirt, it works great. If I go onto a road that had very little snow, its like it pops into neutral, no gear will work, unless I go to a full stop then hit the

4 wheel drive button. It sometimes makes a tiny grinding noise too, when I go back into 2 wheel. This is at a complete stop. It never did this till last winter when my 4 wheel drive shift motor on the transfer case crapped out. My 4 wheel drive wouldnt engage, once the mototr was changed worked well again, started slipping into that neutral right after? Any help would be great.