GM 14-bolt FF Rear End with 149k miles

I used to have the same rig as you and BenK. I replaced the fluid with full synthetic when I bought it used as it had some noises also. I also had not one sign of a leak after many miles of towing somewhat heavy and in the mountains, and I used plain old Permatex sealer.
However I don't think you'll have a problem with what they did, especially with the weight you're towing.
Also, it's not a bad job to do yourself.