If you really want to get into it, first crawl under you rig from the back end (take your camera/phone). Look for the White sticker on each of your axles. It will have the make, model and serial number of your axle.
Go to
www.dexteraxle.com, download your manual. It will be either 7K lb or 8K lb axle.
https://www.dexteraxle.com/Resources...mily=&keyword=
It has all the specs for your axles. Bolt torque, grease recommend etc.
For a grease gun I recommend a 14 oz (standard size, NOT mini) pistol grip (pump with one hand) and a locking nozzle with a quick release (stays on, easy to release).
https://www.amazon.com/dp/B07SVGHNPV...ing=UTF8&psc=1