Thanks for all of the great suggestions and also for the related discussion. To all of you that suggested that I may have a tire structure problem rather than a balance issue - you were correct! I had my local tire store take off the tires and the attached pic is what was found. Probably good that I didn't try to get one more trip out of them even though there was more than 50% tread remaining. I will now adhere to the replace them by time instead of wear approach.
If the vibration is so bad you can feel it, I would be carefully examining those tires for broken belts, slipped belts, or blisters. Do it before you tow again.
Thanks for this and all of the responses. Based on the comments I'm starting to think that I do indeed have a tire structure problem rather than a balance issue. I stated in my first post that the tires were original (2009). Having thought about it I don't know that this is true but they are at least 5 years old.
I'm going to take them off and drop them at the local tire dealer for inspection.
I have a new to me 2009 Komfort TT, 24', towed by a '13 F150 Ecoboost. The trailer has original tires mounted on factory upgrade aluminum wheels. I have put on approx. 3500KM. During the last trip I started getting a vibration starting at 80 KPH and continuing to 100 KPH. It noticeably smooths out after 100. This seems to be a wheel balance issue. To narrow it down I unhitched the trailer and drove the truck without any other changes. Vibration was gone. So...it seems like it is the trailer. I understand that there are varying opinions on trailer tire balancing. Mine have not been balanced. My questions are; Can a tire vibration on a trailer be felt in the tow vehicle?, and Does anyone have experience with the Dyna Beads product? Thanks in advance.