I have gotten into the habit of putting jack stands under the rear corners of the trailer before driving (or winching) a vehicle up. This prohibits the trailer from going down under weight - thus keeping the tongue from jumping up (and potentially lifting the tow vehicle's rear wheels). In extreme cases, I'll put stands under all four corners of the trailer until I'm all secure - then with a floor- or bottle-jack, I'll ease each corner up and remove the stands. It takes only a few extra minutes, and I'm sure that my trailer isn't going to be tipping or jumping unexpectedly while I'm trying to get the vehicle spotted and secured correctly. Chocking the trailer fore and aft of the tires will add security also!
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