Sorry for my delay in updating my post. Here are the facts now --
1. The vehicle would not go into 4WD low. The P73 software update was done this past Spring.
2. The mechanic said that a variety of codes were showing, all with respect to transfer case operation (he says that he has a full Jeep Chrysler diagnostic code reader). He wanted to do further diagnostic tests and take a closer look at the transfer case but he first contacted the extended warranty company for payment authorization. The warranty company sent an adjuster out who said they would not pay for any more diagnostics but would pay for a complete replacement of the transfer case instead. A rebuilt transfer case was ordered and delivered about one week later.
3. My mechanic put my Commander on the lift and started to take off the transfer case. He said that he noticed that the wiring cable to the transfer case appeared to be incorrectly located and was being severely pinched.
4. He said that he repaired the wiring cable and relocated it. He then tested for codes and nothing came up. He had another mechanic drive the vehicle about 60 miles and also use 4WD low, and everything appears to be working properly. So the wiring cable seems to have been the problem and not the transfer case. He said that he could have replaced the transfer case (the warranty company would have paid for it), but he thinks we are better off with our existing low mileage (about 30,000 miles) transfer case than a reconditioned one (also, "if it is not broken don't fix it"). If we have any more problems, he will replace the transfer case if needed.
5. My wife got the Commander back about one week ago, and has driven it about 100 miles or so. No problems, including with 4WD low. The only issue is that we were without the vehicle for about three weeks.
6. Gas mileage still is lower than it has been, but the mechanic believes that it may be from the winter gas formulation where we live (in the Northeast US), as other customers are making the same complaint now. He said that the vehicle is not producing any codes, and all maintenance is up to date, including a spark plug change just two months ago.
Any thoughts would be appreciated. The problem seems to be solved.