Eric,

Kind of reminds me of an old Dodge 1 ton van that I used to have. It was completely empty in the back and I used it to haul debris to the dump. On a major overhaul the crew fully loaded it with quite a bit of an old masonry chimney and some heavy wood.
I got to it later and it certainly looked overloaded. Went to take it to the dump (about 5 miles). Made it about three of those miles and I started to hear a very loud noise coming from one of the front wheels. Tried to get back home with it, but to no avail.
The front wheel bearing exploded in an intersection and I just made it to the side of the road. Had it towed home.

Replaced the bearing and made the trip again the next week.
When the van was finally weighed on the scale, it tipped the scales at about 7500 lbs. The empty van was about 4000 lb. so the payload was 3500 lbs. Fortunately it was a 1 ton van, but, was carrying nearly twice the rated load.

I'm sure that you probably did nothing to the M/B. But, remember that 1200 lb. in the trunk adds about 1400 lb. to the rear axle and unloads the front axle by 200 lbs. M/B has a rating of 900 lb. for the vehicle that is equally distributed.