I never felt it when towing the .21 Special but now towing the Baja I feel it.

I have a full size Ford Bronco 5.8 with the tow package. When around 60-70 mph I get an intermitent vibration like on for a second then off for a second. I don't seem to get it when I coast or at slower speeds or not towing.

A friend of mine with a 3/4 ton Ford long bed extended cab said he had the same problem and it was the rear end, something about clutches??? sliding in and out?? He claimed there is some additive to put in there. Anyone else expierience this or know what it is?

Simple solution

Drive slower than 60 or faster than 70

Problem solved

I used to work at a Ford Dealer. Run into that concern couple of times. Yes, Rear Differential is one possibility but also transmission is another.
Another vibration I found on mine a couple of years ago turned out to be one of the trailer tires had a broken belt.

Check the U-Joints as well.

Check the axles on the trailer. I had two vibrate like that and the problem on both was an axle had shifted out of alignment: probably hit a curb. It was an easy fix: one the shackle had flipped upside down and the other the spring shifted on its pad.
Due

Goes away on coast?

U-joints.

How much does it squat the truck when hooked up to it? Could be drive line angles. Check the easy stuff first the the truck over. Check the U joints for slack as well.

Thanks for thee replies. The truck doesn't squat much. When KCHOTBOAT and my self went to Chicago to pick it up with his 1 ton we didn't feel it. If it's u joint's wouldn't it do it all the time?

Oh one more thing, When we brought it home the buddy bearings were fine but I have thrown the same one twice going to the lake since.

U joints will only show up under a load. Another possibility is if you are towing in overdrive, you might have a lockup transmission torque converter problem.