I have a somewhat stock 1970 Bronco. I have a mild lift of 2" and 30" tires. They are Bridgestone Duelers with about 5K on them. I drive the truck a few miles every week, but not too far.
Last April I drove about 200 miles on the PA Turnpike and noticed a vibration at speeds from 50-65 and it seemed to be only when I was under power. Coasting or slowing down did not seem to cause this shake. It seemed to be coming from the rear.
In May I purchased a new driveshaft and installed it. This included new CV and U joints. A lot of the vibration is gone, but yesterday when I took it on the highway for the first time I noticed a vibration that was rather severe (I think different from the one in April). No noises or grinds, just a rear end vibration that came on at same speeds as before. What gets me is that it seems that the vibration seemed to less noticable on the way home. I was just wondering if someone else has had experiences like this with a severe, but not consistent vibration.

A couple other notes, up to 50 mph it is smooth as glass, as well as if I run up to 70 it smooths out as well. I am not comfortable at those speeds though.

I looked over everything when I returned. No leaks or loose lugs.

Tires?
Wheels?

Ideas?

Thanks a lot everyone.

 

if tires are ok, could be bearings in transfer case,,,

 

That is interesting, I had not thought of that...
Is there an easy way to check them?

Thanks

Sporto

 

The easy way is to get a dial indicator with a magnet base. Put the indicator base on the transfer case and put the indicator on your pinion output. Grab a floor jack and place it under the pinion (NOT the driveshaft). Jack it up very slowly and easily, you are by no means jacking up the truck, it should only take a 1/8 stroke of the jack handle to see if the bearings are indeed gone...you should't see any more than 5 thousandths of an inch (0.005). I'm guessing at the distance. If you see 10 or 20 thousandths (0.010 - 0.020) it's time for an overhaul.

 

Thanks for the reply. I see what you're saying, the good news is that I was planning on getting a rebuilt TC and 3spd next year, maybe I will move that up a bit. The TC is the only original untouched drivetrain component left. I replaced the gearbox with a junkyard model about 20 years ago, and the 9" has been rebuilt once as well. I enjoy driving the rig too much to not make the investment... Thanks a lot for your help guys, I will update you as to what I find out.

 

Check your u-bolts. Vibration at that speed could also be pinion angle, but it doesn't make sense that it would appear suddenly after no modifications.

J.D.