1998 Chevy Blazer PROBLEM SWITCHING FROM 2-WHEEL TO 4-WHEEL

My 98 Blazer has just started having problems switching from 4-wheel hi to 2-wheel drive while driving. When I push the 2-wheel button, it continues in 4-wheel drive for another minute or so until it switches.

When it does, there is a jolt and a noise that feels and sounds just like I hit a pothole. In fact, that’s what I thought had happened the first few times this occurred. After this jolt, the 2-wheel drive is engaged.

I've tried stopping and putting it in park before switching and, although this did work some of the time, it did not every time. Another thing is that, during the minute or two it takes to switch, the truck will tug slightly to the right. Just like it would if it was slightly out of alignment. However, after it switches, the tugging stops.