l have a 1986 k5 blazer and it is in 2H, when l put it in4 wheel drive gear shift, (N)eutral it started to grind and struggle to drive, until l stopped and popped it back into 2H. Is this correct or do l have a broken 4wd gear shift?

You should have 4 wheel high, 4 wheel low, 2 wheel high, and neutral
Try shifting through all of your 4 transfer case positions until you feel comfortable. You should be able to feel all 4 positions and be able to
tell the difference

If u r trying to go into 4-lo there r a few steps u need to take to reduce where & tear on ur transfer case.1 stop & put the truck in neutral,2slam the 4x4 selector into 4-lo(do not stop at the neutral position in between 2-hi & 4-lo) 3 put the truck in reverse & back up about 4 feet so u know the 4x4 if fully engaged.4 put the truck in drive but do not exceed 30mph in 4-lo.

EDIT-The 4x4 selector should be in 2-hi to be off..never try to drive with the 4x4 selector in nuetral.