Four wheel drive issues

So I apologize that this may turn into a really long question. I want to give as much information as possible and this site has been super helpful in the past. My vehicle model is the Sport Trac (on the options to begin this question Sport Trac was not on the list) it has four wheel drive. The manual says the four wheel drive can be engaged while moving so long as the wheels are not spinning. It has a simple turn style selector switch. You just turn it from two wheel drive to four wheel drive high to four wheel drive low. I understand that switch to low requires me to stop and so on. I have never needed four low. Okay, so the problem is that when I engage four high and drive every so often it feels as thought the driver's side goes through a pothole with no pothole present. It sort of jumps and flanks and bangs but only for a second again, as though you have hit a pothole. It seems to do it every fifty to one hundred fifty feet. Now if I stop, put it in park then engage four high it seems to do it less often than if I engage it at say five mph. But none the less it still does this pop bang routine. Furthermore when I disengage four high back to two wheel drive there is a tremendous bang that happens within a short distance especially if I disengage it on the move. After I disengage it, on the move, within usually fifty feet and under acceleration it hits hard. If I am the other hand I put it in four high while in Park and take it out in Park there have been times it doesn't do it at all. But those times are usually when I only need the four wheel drive for just a few feet. Also if I am say going down a hill and do not use the accelerator I do not notice it disengaging. So, it boils down to this, I deliver mail with this vehicle. I love in the Appalachian mountains with a good bit of snow in the winter. I have had a mechanic look at it. First time I had the axle replaced a little more than a year ago but those guys did not really work on four wheel drive. So a couple months ago I took it to another place that does just about everything and they changed the fluid in something unfortunately I do not remember which fluid, but they said it was clean and that my axle was being chewed up again. To me it seems that when I engage the four wheel drive it is not being fully engaged which is why it may be popping in and out of four wheel drive, but again just the driver's side. And also it may not be fully disengaging when asked until it rolls for a few feet. I will try to find out the fluid they changed, but I would imagine anyone who knows about this problem probably knows the fluid in question. Anyway, I cannot afford anymore exploratory costs to find the problem for a couple months. Any ideas would be helpful. I have basic knowledge but no special tools. Thank you in advance.