Hi, I'm trying to help my brother out with his 1999 Jimmy with the 4.3L, as he is stuck without a car. The problem is that it will not shift into gear - Drive or Reverse. Any idea on what is causing it? More details:

It was running fine, and stalled when he pulled into the driveway. The next morning he goes out and it wouldn't budge. I found it to be charging fine at 14v running, but it had a bad battery. I was thinking that this could have been the case, since a bad alternator/battery can cause shifting problems.

With a new battery it starts/runs fine but will still not move. The shift linkage is working, and will go into neutral fine. It has electronic 4wd. 4wd won't engage either, the button just blinks and goes off. Tranny fluid is full and smells/looks fine.

Note: it has been very cold, with -40 windchill some days.

Any help would be great!