Just a thought here: If you only have one roller on that rear truck, you might be able to rotate the roller assembly plate around 180 degrees so the the roller is "Trailing" not 'Leading". It's like "push vs. pull". Now you are pushing the roller forward, but if reversed, you would be pulling the roller thru the crossover. The assembly just pops out from the truck side frame.
You say that there is no spring over the roller, but that type of roller plate should have a copper/phospher bronze piece of metal pressing down on it. It can be adjusted to have less tension on it, but caution says to be sure that there is enough to have pressure on the roller and good track contact.
Another "fix" might be to relocate the roller plate to the front truck, the end by the drawbar that the loco hooks on to. That way, again, the roller would be trailing and the tension on the drawbar might exert additional pressure on the truck to keep it from jumping. If that is decided, you may have to unsolder wires on the inside and extend them if not enough wire is available there. Hope this might help. Dennis M.