2003 e150 vibration sound when accelerating

Alright, so I own a 2003 e150 tuscany conversion van. Recently we went on a road trip towing our camper from IA to SD to WY to CO. While we were in SD our van started making a strange vibration noise that sounds like its coming from the back end. The noise would best be described as 2 tin cans vibrating next to each other. The performance of the van is not affected its just a strange noise and I was wondering if anyone knew anything about this or what it could possibly be. The noise only happens when accelerating to about 10-15mph but stays constant at approx. 12 mph.

Also we just had it taken in before we left to get the rear AC working again since one of the seals was cracked and not recirculating the air. The rear differential was also leaking slightly so we had that fixed and we had all the fluids changed out/brakes redone. If that helps any.

Thanks in advance. :)