IMHO it sounds like a u-joint gone bad, Have you looked at the drive shaft?

 

I just had the two-piece driveshaft rebuilt by Inland truck parts. They replaced the slip-yoke, the spline shaft, and 2 out of 3 of the U-joints. They tech there said that the differential U-joint didn't need replacing. He also said it balanced very well. I greased everything on the driveshaft including the splines. I lined the arrows up on the two different shafts and double checked that the u-joints were in phase. I also replaced the U-joint U-bolts and nuts and made sure the bearing caps were inside the little tabs on the tranny yoke and the diff yoke. I'm putting money on a bad tire in the rear. The tires are pretty old with some minor surface cracks starting to show in the sidewalls. Plus, there isn't much tread left on them. I'll probably replace them pretty soon here.

 

Getting large tires balanced can be a pain in axx. Ask the GY tire manger what the balancer zeros out too. This is how close or out with no tires on it. Idel is with in 1/8 zo. If it's 1 zo. like most places this is not real bad with smaller tires. Big 4 zo. into 4 tires is not real good for speed over 45 mph. But tighten up that front end this may help reduce some of that shaking Plus check those king pins and all the other posted things that were listed...my 2cents
orich

 

I replaced all the bushings, the front springs, the front wheel bearings, and all the brake hardware in the passenger front drum brake and it's a lot better. It doesn't quite feel like the truck is going to completely fall apart, but it still shakes a bit. After I get all the Christmas stuff together for the wife, I'm going to run it up to speed with the rear wheels on jackstands. If I see vibration, I'll take the wheels and tires off and try it again. If the vibration goes away, then it's new tires. If not, I don't know, I guess I'll keep looking.

 

Hmmm. I've learned something now. Yesterday, I had the spare tire balanced. I took it and replaced one of the rear tires. Test drove it...no change. Switched the spare to the other side, test drove it, no change. That still leaves the possibility of it being both back tires.

Next, I jacked up the rear and put it on jack stands. Ran it up to 75 or 80 and the only change was that the vibration started as early as about 30 mph. Then, I took off both rear tires, put the lug nuts with washers on to keep the drums on tight, and took it up to speed. The vibration was much worse and was there from about 15 mph and up. I went around to the passenger rear and it looked like the axle was jumping up and down. It didn't look bent, but there was a ton of slop in the bearing. I shut everything down and tried moving the axle by hand. It had about 3/16 of an inch of play in and out. I couldn't really get it to move up and down or side to side but there was definitely play in and out. The driver's side was tight. I knew I had a bad bearing that was giving me the thrum, thrum, thrum sound, but I didn't think that it would cause a shimmy that felt like a tire out of balance. Of course, it could still be a bent axle. I'm assuming that the machine shop should be able to check that.

I guess that means my project this weekend is pulling the axles and replacing bearings. Oh well, I needed to count the splines on the axles anyways.

One queston...is there a bearing race in the end of the axle housing that will need to be replaced? I just replaced my first set of front wheel bearings last week and I have all the proper tools, but I just haven't been in there yet.

 

f-100 & f-150 have a press on rear bearing. Should be no axle end play more then a few thousands..
orich

 

Any idea how I can tell which bearings I need? The local parts store wants to know ring gear diameter and axle rating. I can't find either.

 

just pull the axles out take them to auto shop. They have to be pressed off and onany way. They can match the beraing ez ...my 2cent
orich

 

I already had a shop lined up, but they told me that I would need to bring the bearings. I'll call some more shops tomorrow to see if they can get bearings for me.

How can I tell if I have a 3300#, 3600#, or 3750# axle?

 

theres a number on the bearing. damn auto zone should have or can order it.
Take the axel to another shop that can replace it. I didn't notice your truck model, if it's F-100? it shouldn't be over 3300lb... You can most likley find it on ebay from the bearing name and number on it....
orich

 

Is the differential open from the back or does it have the 3rd member have comes out the front???And what model F-???

 

Well up until lunch hour today I had only pulled one axle and it didn't have any numbers or markings on the bearing at all. I pulled the other one during lunch along with a seal and the other one does have numbers as does the seal.

The diff is a ford 9". It's technically an F102 according to the vin plate. The axle code says 02, which, according to my sources is a 3300# 3.00 ratio open diff. The tag on the diff led me to believe that is actually a 4.00 open diff. I do know that it is definitely an open diff. I'll crawl under it this evening and re-decode the diff tag and count driveshaft revolutions to make sure.

 

Also be a good time to flush out the Diff. with axle out you can even clean the axle tubes too. Pull the cover. Get a few cans of quick start spray cans or brake clearner spray. let drain good wipe inside bottom to get all the sludge crap out. Spray until it runs out clean. This is what, I have done for the pass 40+ yrs plus get a drain or filler plug that has a magnet in it. Plus put one in the tranny pan,engine. I even place one in the oil filter. there, In there, I use the megnet strips from old frig door. I just use a piece about 3" long.. Never had any troubles doing this .. my 2cents
orich

 

There is no cover...I need to pull the third member to do it. I'm sure I've seen some 9" diffs with a drain plug, but mine doesn't have one. I'm seriously contemplating it, but I'm also planning on replacing the third member to get rid of some slop and get some higher gearing plus it has a leaking pinion seal. My thought is that I can refill the diff and run it for a few months until I find a good third member with say 3.25 posi in it and then clean it all out. Am I wrong on this one? I'd replace the axle seals when I replace the third member too.

 

Yep! no reason to do it twice..