I'm having problems with my 4x4 on my 87 chevrolet. When I am at idle I put the trans in neutral and engage the 4x4 high. I can drive a short distance and it kicks out of 4x4 and the truck just revs like if its out of gear. I have to come to a stop to put it back in 4x4 high or 2wd. Also when the trans is in neutral it will go in 4H only no 4L. Any info will help out.