Sign of a sick snake?

My snake shed about 1/3 of its body about 4 days ago (maybe 5). The other 2/3 (tail end) is faded in color and has some loose skin but most of it is on there like normal skin. I tried putting her in a humid box, soaking her, having her struggle through moist paper towels, and feeding her to loosen the skin. So far, I've seen some skin get loosened but the faded color persists. My temperatures are normal (low 80s on warm, mid to high 70s on cold) and I've been keeping up the humidity since she had trouble shedding. She has rocks and tree bark to rub against. She also has several hide places on both warm and cold side, and I use aspen bedding. Ever since she started the shed, she has been avoiding the warm side. If I turn the heat pad off, I will find her on the warm side (after it has cooled down) and if I turn it back on again, she will return to the cold side. Could it be that my snake is sick?

Have you double checked the temps on the warm side? I originally had my thermometer about an inch above the substrate - when I put it down into the substrate over the UTH the actual temp was 110 degrees. (Mine had been doing the same thing - avoiding the warm side.) You may have already thought of it, but just in case...

substrate temperature reached 91 deg

From what I've read, that's not too bad, but may be a bit warm for her. If you can get one of the inexpensive thermometers with the probe (I got mine at Wal-Mart for $6) you can put the probe down in the substrate and continuously monitor it.
I'd see if you can get a rheostat / thermostat to regulate the UTH and keep the temps in the mid 80's on the warm side and see if she does a bit better. Not sure if thats the problem or not, but it couldn't hurt.

the problem is she hasn't completely shed despite turning up the humidity.

does my snake have internal parasites?

It's unlikely she has parasites, so don't worry just yet. This happens from time to time with most snakes, and usually the cause is low humidity. Suddenly increasing the humidity isn't going to make the rest of the skin all fall off though!

Please see your other thread for tips on what to do now, and good luck.