Since you have a fairly light trailer, just put in D and go.
If you find the transmission downshifting a lot (dropping a gear, then going back up a gear, then dropping a gear again... and continuing this process), use the "-" to downshift and hold it in that gear. If the transmission still hunts for a gear, use the "-" to drop another gear.
Letting the engine run at a higher RPM, in a lower gear, is better for everything.
The "+" and "-" on your Jeep allows you to restrict the transmission to which speeds/gears it can use. You're setting the "ceiling". When utilized, the transmission will operate normally, except it won't go into a gear higher than you set with the "+" and/or "-". Try it out anytime, towing or not. You'll figure out what it is doing pretty quick.
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