My 1988 suburban has 10 bolt. There is a gear noise that is most evident at between about 25 to 35 mph under light load, if I let off the noise goes away. Any ideas what it is ??? It is not real bad yet, but I am planning on towing this weekend, about 3000 pounds into the mountains and dont want a real problem. I am thinking I will change differential fluid before I leave. I use synthetic in my 1999 Ranger, but have always used regular lube in the old vehicle. I am wondering if maybe it would be a good idea to put synthetic in the burb ??? I haven't in the past because I was afraid it might start leaking. Thanks in advance for any ideas/advice.