Chevrolet Repair: Rear differential question, rear drive shaft, differential question

Question
"Hello. My mother decided to drive at high speeds while inadvertantly in 4wd. Now my truck (97 Chevy 1500 Z71 5.7L V8) seems to almost be stuck in 4wd. When I turn left, the Left rear wheel seems to have extra torque and is making the tire go screech, screech, screech as rapidly as I accelerate. I've narrowed it down to the rear end by removing both drive shafts to see if transfer case shifted ok and if front diff had anything to do w/it.Both were fine. Then removed rear diff cover and saw no obvious damage or excessive metal shavings on magnet, but I am not familiar w/Diff's and Haynes manual info was very limited. I attempted to remove the axle's but was not able to remove the "c" clip holding them inside the diff. I stopped there as it was getting dark and the auto parts store w/my diff gasket was closing. Is the rear diff on my truck a full time locker or did something get seized in there permanently locking it? Does the diff have a limited slip? If so, how do I fix that? Where do I find it? There was plenty of fluid in xfer case, front n rear diff, and tranny. The xfer case made a loud clunk w/rear drive shaft attached while shifting from 4-hi to 4-lo. W/O drive shaft, xfer case was smooth. It is an electric (pushbutton) 4wd shifter (no lever). I feel as though I am close but just need that little extra knowledge that only comes with experience. I thank you in advance for any help you can offer.  

Answer
The first thing to do is just change the fluid and add the additive to the rear axle. This might just clear everthing up for you. There isn't much that could happen after your mother did that. Let me know how you make out.

P.S.   give it awhile after you change the fluid

Mark