Ok, no extreme uses then. Your axles aren't actually very fussy about the gear lube or exact viscosity they run on, the Gear Lube only needs to be GL-5 rated which 99.99%are. Since we don't know if your rear axle has the Tracloc limited slip differential, for safety's sake let's assume it does since the Tracloc needs a particular additive called a 'Friction Modifer' to work properly. The good news is that 99.9% of the GL-5 gear lubes you find on the store shelves will already have that required friction modifier additive which is also compatible with axles that don't have a Tracloc.
Since you do plan to tow a trailer, go for a slightly heavier viscosity GL-5 gear lube like 80W-120, 85W-120, 90W-140, etc. Those are all very close to each other in viscosity so don't fret over which of those to choose.
To verify the gear lube you buy has the friction modifier additive, look at the bottle's back label. If you find words to the effect 'Compatible with limited slip differentials', it has the friction modifier additive so that would be a good choice.
Any of the major brands of GL-5 gear lubes will be fine. I use Valvoline, Torco, or Currie, but Castrol, Mobil, Pennzoil, Havoline, Walmart (lol yes), are all fine too. There are no major brands of gear lube I wouldn't run. I also prefer a conventional gear lube in my axles but synthetic will work fine for your uses too.
Draining it is a no-brainer. Use a 1/2" socket wrench to remove all of the bolts of the differential cover except for the very top bolt. Leave the top bolt loosely threaded in so it will hold the cover and prevent it from falling into the old gear lube catch basin when you finally break the cover's seal against the axle housing and the old gear lube gushes out. Leaving that top bolt loosely threaded in will prevent it from making a huge mess in that catch basin if it fell in.
The cover is held to the axle housin by RTV sealant which has a tenacious grip on the cover. It will take some prying to get it off. I use a sharp bladed screwdriver & small hammer to insert between the cover & housing to pry it free. The cover won't come right off but it will come off with a little patience.
Once the cover is off, inspect the gears inside for obvious damage. You can flush out old lube & metal particulates with brake cleaner.
To replace & reseal the cover, use a wire brush to clean ALL (!!!) of the old RTV off the two surfaces. Make sure both surfaces are bright & shiny clean so the cover won't leak later.
The easiest way to seal your cover is with a Lubelocker gasket LubeLocker - The Ultimate Drivetrain Gaskets
which is for sale all over the Internet. You only need to know if your rear axle is a Dana 44 or Dana 35 so you can choose the correct gasket. Or use Permatex Ultra-Black RTV to seal it, paying close attention to the directions on the tube.
Good luck with it.