Originally Posted by Uniblurb View Post
Downhill isn't even necessary. If the balance is far enough out of whack it'll do it on flat land. With a trailer almost in good balance, the little bit of trailer push on a downhill can be enough to start the wagging oscillation. As can wind; higher speeds; applying the brakes; a quick manuever such as an evasive to avoid something in the road; rough expansion joints, or even merely changing lanes - particularly when there's a grade difference between them such as where a merge lane meets the highway. People get into problems when they've been cruising along fine just riding the edge of wagging and then suddenly one or two of these other things factors in and push things over the edge.
In cases of badly off-balance trailers, once the oscillation starts it can be impossible to stop and instead quickly get worse and worse until the trailer tries to pass the tow vehicle and they both end up on their side. Using the brakes of the towing vehicle and a trailer w/o brakes results in the same trailer push as a downhill grade does.
I'm not sure how much a donut spare weighs, but you're not looking for a whole lot to get in the comfort zone. Anything moved from the back of the XJ to the front of the trailer will decrease the load on the rear suspension thus increasing steerability and also improving trailer balance. I've run with too little weight on the front tires before too and don't like it at all. Every little bump makes it want to float and wander as the suspension bounces weight on and off the steering tires. I hate to ask but have to wonder what the weight bearing capacity of a donut spare is. I guess if need be it could be installed on the front axle where the load is significantly less in order to limp to a tire repair place.
Pulling a good load can be more difficult than many people realize but I think you're really close here to getting it dialed in, which makes all the difference.