Does anyone know about the workings of 4x4's I'm totally at a loss?

A month ago or so I noticed that the (service 4 wheel drive) message appeared o n my 03 Tahoe's l.e.d readout. I really didn't think much of it since manufactures have warning lights come on at different mileages and this coincided with 100,000 miles. It did it a couple more times and I talked to a mechanic while getting my oil changed and he said the oil in the differential was probably low. Everything was good for a couple of weeks and when I would start it I noticed it was in 4 low or maybe 4 auto. It would go back into 2 high though. Then I noticed that the neutral light was on while I was driving. I read in the owners manual that that light should only be on while towing the vehicle. I took it to a shop and they told me the differential was communicating fine but the front pinion seals needed replaced and they replaced them. Everything was good for two days and I it started in 4 low this morning. Does anyone have a clue? Please help