I've pulled a 3,500 lbs trailer off-road, but not with my RL. If we can assume our RL would pull the trailer the same off-road as an old Sierra Classic, then I would say it depends on whether there is any soft sand you have to pull it through. If there's sand, it would be easy to get stuck. Since you have some tricky off-road maneuvers to make, I would avoid using a trailer.I like the idea except for the off-road portion... any experience with a trailer off-road? I've got one particular 90deg turn into a steep grade with a meandering rut that I worry about in particular. ...
One thing you could try to keep the rear from sagging so much is distribute the weight a little more. If you were to follow the owner's manual --which I don't always do in these cases-- our RL's have roughly a 1,500 lbs load capacity (that includes driver and passengers). 1,100 lbs of that load can be put in the bed leaving 400 lbs that you can locate in the cab. If you weigh 150 lbs, consider putting only 17 bricks in the bed and flip up your rear-seats and put another 5 bricks on the rear-cab floor. That may help distribute the weight a little while keeping you at the manual’s published load limits. Of course you could reduce the number of bricks in the bed further by relocating even more bricks to the rear-cab --as well as the front passenger seat floor-- for even more weight distribution.
Given where the high-strength steel crossmembers are located in the bed, I would put more of your bed weight forward (near the cab) and less over the In-Bed Trunk for safety.

If you're thinking of putting any bricks in the In-Bed Trunk, keep in mind that it has a weight limit of 300 lbs. Given how that trunk is suspended around the frame, I would not want to overload that storage compartment.
