Toyota Repair: Front End 4wd, driveshaft, actuator

Question
96 T100; ex cab; 4wd; 3.4L; non ABS; 101,000 miles

Something is wrong with my 4wd. When the truck is put into 4wd the front wheels lock up completely, they will not spin. For instance, I was driving up an icey hill and put it into 4wd and the front tires locked up, and I slid back down the hill. It is also a pain to get it back into 2wd so the front tires will "un-lock". I am hoping it is not the differential as that is very spendy. So other then the differential could it be a front cv joint or front drive line U-joint? Is there a way to tell what it is?  Also If I turn a corner sharp and on a slight hill or hit a bump there is a very bad cranky/grinding sound from the front which also leads me to belive the problem is related to a CV.

Thanks, I hope this is enough information, but if there is any thing else you need to know I should be able to tell you.

Answer
The front driveshaft is turning all the time so is the differential, the way 4 wheel drive is engaged is with an actuator on the front which locks the axles to the differential, if there was aproblem with the differential the problem would be there all the time, the problem is external to the differential.