I had to have my differential rebuilt. I jacked it up in the rear, started it, put it in drive and the rear end was making a swishing kinda sound. I pulled the half axles and replaced the outer bearings (labor intensive but, cheap) and that didn't fix it. So, I took it to my mechanic and he replaced everything back there except the hog's head. Not saying that's your noise but, it's worth checking. If it is shop around for the price on a repair.
Sumo Supersprings are pretty cheap and an easy DIY job. I highly recommend them to flatten out the ride. It took me about 20 minutes to do the left side and about 5 to do the right side.
You don't need to pull the grill to see if you have the GM transmission oil cooler. It's located dead center and is over the bottom half of the radiator. You can see it in there without pulling the grill, although that's a super easy job. There's a youtube video on it. Heck there's a youtube video for almost every possible repair on, Yukon, Suburban, Tahoe, Sierra, Silverado.. I may still add the extra cooler but, I have already towed the RV a number of times and never noticed that the transmission was suffering. Based on the mileage and looking at the hardware I'd make an educated guess that mine has been rebuilt. It's tight and runs great.
When I bought my Burb it had a new-ish engine (less than 50,000 on the engine according to the dealership). I saw that the exhaust manifold bolts, water pump and alternator were brand new. Nevertheless, I sold my Honda CRV and made the Suburban my daily driver. My mechanic likes the 1999.5 - 2006 GM big suvs and trucks.
The biggest thing you need to know is don't lug that engine when towing, it hates it. It doesn't mind RPMs though. Those engines don't have a lot of low end torque, especially when towing. But, in the mid and upper range they pull strong. I pull mine down into 3rd unless I'm on a long, easy, downhill grade. I haven't noticed that the gas mileage suffers all that much to be honest. I'm sure it does suffer some but, not really that bad.
Have fun. I'm so ready to get back out there and take some trips.
