Have you tried lifting the front tires off of the ground with the loader? A month ago or so I was having some problems getting my 7520 in and out of 4x4 mode, lifted the front tires up with the bucket, went in and out without a problem. I then figured that something was putting a bind on the system. I checked air pressure in all the tires and found that they were a bit soft. Aired them all up to max pressure and wala, no more problems getting in and out of 4x4 mode. With the tires not being the proper inflation, the diameter was wrong and putting the system in a bind. Proper inflation took care of the problem for me.
This may not work for you, but it is simple to try.
Good luck.