2003 Ford Ranger 4WD Problems

In 04, I bought a new 2003 Ford Ranger 4wd. Around 20,000 miles, when I put it in to 4wd with the onboard switch, it would start to lurch, etc. About every 1/8 of a mile. There is not really a noise, but it does cause me to lose traction. It went in on warranty and Ford replaced the transfer case. They thought this would fix it since the splines were totally screwed up. That was in February. I didn't use the 4wd for about a month, but when I did, I was towing 2 snow machines and it started the problem all over again. I was told that we couldn't really recreate the problem since there were dry roads in my area, so I had to wait until this winter. So, now I'm dealing with Ford about the transfer case again. The truck has 40,000 miles on it and when put in 4 wheel drive, it will lurch or jerk. When the onboard switch is put back in to 2wd, it will jerk once then is fine. My opinion is that it is the computer chip, which I believe they have to run the transfer case. This situation will ruin my transfer case as it did the first time. Now they are talking about my tire wear, which could void the warranty. This lurching was a problem since day 1, and is the same problem this time. My question is have you heard about this problem before and what may cause it. Because my dealership is quite confused. I live in Alaska, so 4wd is quite important to me. It seems to do it worse on hills or when I am towing, but it does it flat also. When it did it on the hill, it caused me to lose traction which is a huge safety issue. So, if you can help, I would appreciate it. Thanks, Phil.