Just a few thoughts, have someone watch you (or video you) backing down an incline, and see what is happening. If it's the shocks, I would expect the observer/video to see them move. Another area to look at is the mower deck (you don't mention if you are mowing or not when it happens). I've noticed my rollers tend to dig in a little in reverse in very soft ground or if there is a protrusion (grass here, not there) that the end of the roller can hook on, especially if I'm turning. I mow my grass near the top of the cutting range, and the rollers are sticking almost straight up and down. After that I'd look for something loose in the suspension, check both ends of the control arms and top and bottom of the shocks. It seems to be a relatively simple suspension system, so an issue shouldn't be too hard to spot. Try adjusting the shocks and see if you can find a setting that helps or hurts the issue.
Is it one particular area of your yard this happens? Have you been able to demonstrate it to the dealer, or just told him about it? Is the deck up or down? Are there bumps or is it smooth? Are you coasting, or using the reverse pedal? How steep is the incline? On my Conquest, rapidly taking your foot off one of the go pedals (either forward or reverse) has a tendency to act like putting on the brakes. This is common with many hydrostatic transmissions, such as in forklifts. If you are on a steep incline, gravity could be accelerating you to a speed faster than the hydro thinks it should be for that pedal position, and slowing you down. I know very little about what goes on inside one of these units, so I don't know if it could be adjusted in some way or if a fluid/filter change might resolve an issue of some passage getting blocked.
Check with another dealer or two in your area. Whatever the issue, you probably aren't the only one in the world it is happening to, the more people you ask, the more likely someone else is to have seen it. They may or may not have correctly diagnosed it and solved it!