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My 74 was doing the exact same thing. I had removed the rear driveshaft while towing it, and reinstalled the driveshaft "out of phase". I assumed the driveshaft sections were balanced individually, but nope they are balanced as an assembly! So, make sure your u-joints line up. If so, turn it 180 degrees and try again. Good luck!
So I swapped the rear and front tires around and took it out and unfortunately it's still vibrating between 30 and 40 mph. It did seem to get a little better but was definetly still there.
After swapping the tires and not noticing any difference I had someone rock the steering wheel back and forth while I observed the suspension components. Everything seemed to be pretty tight. I'm by no means an expert but everything seemed tight and I didn't see any obvious play in any of the bushings. I also greased everything while I was down there. Not even sure the last time everything was greased but guessing multiple years ago.
The more I read I'm thinking my next step is probably to address the driveshaft u-joints and balancing. The shock absorbers are also old so thinking about changing those out as well. I was going to start with the shock absorber first and then go from there. Curious what people think of this plan? Thanks so much!
Yeah, that's what I'm thinking too. I'll probably still look into the driveshaft as well just to make sure I'm not masking a bigger problem with new shocks if the shocks does help/solve the issue.
Tire swap a free troubleshooting step, shocks replacement, if they are worn out then sounds like a next good idea. Drive shaft being "in or out of phase", you can just look at it and tell. Also look for a sign of a flat weld on weight missing. Looks for a paint difference (a small rectangle none painted area) or a short pin looking piece sticking off the d/s. That should have a weight welded to it.
Drive shaft u joint worn, you can ck that with the truck driveline un-loaded. Meaning NO put it in park and it roll a bit and load the drive line. Like park it on flat terrain, chalk the wheels. NOT running, put it in N and un bind the drive line. Make sense? Get under there and see it the d/s ujoints have play. Also ck the yoke/pinion for play. https://www.ford-trucks.com/forums/1...vibration.html https://www.ford-trucks.com/forums/1...t-phasing.html
If a reconditioned and re-balanced driveline was recently removed for any reason and re-installed, there should be new alignment markings to line up front and rear driveline.....I diagnosed a friend's Ford Truck as it had vibration and seen the alignment marks out of alignment with both drivelines.
Ditto on ones that have mentioned a possible shock absorber possibly causing this issue - Especially when the OP stated balanced tires and alignment was done, as I assume front end parts are in ok enough or in great shape.
I had a pair of new premium front shocks on my 70 Thunderbird with a new rebuilt front end done in 1992, and within one year I had experienced the same described vibration issue at a certain speed threshold after sitting in the garage during the winter....Asked the reputable front end shop owner what might cause this sitting all winter and without hesitation replied : - replace those new shocks with better new ones, which solved the issue as the next 28 hibernating garage parked winters I have never had another vibrating issue.....Went from a Sears premium brand and replaced them with new Moog shocks.
When I test drive a potential rig I might buy, I always pay close attention on the sensitivity of my hands on the steering wheel itself, in order to detect any minor or major vibration issues.
As also noted, one of the first inspections I do on these old Ford Trucks on a potential buy are : Frame area of the steering gear, and front cab mounts checking for cracked areas.
So I replaced the front and rear shocks this weekend and the vibration definitely got better but still isn't totally gone. It's now basically just there right around the 37 mph area and is gone by 40 and also has a much "softer" vibration than before. I also checked the driveshaft closely and the yokes are in alignment. I also blocked the tires, put it in neutral, and did a driveshaft wiggle test and none of the U joints had any play at all. The only play at all was slight movement at the carrier barrier but the rubber itself is clearly intact and it appears that it's designed to flex that little bit. I did a close inspection on the driveshaft and saw no signs of weights that had fallen off either. I'm now wondering if maybe there is a front end suspension component that just isn't quite solid but I'm just not versed enough to diagnose it myself.
I'm taking my 2wd rig to a front end man today for a similar problem. I'll let you know the results. My vibration becomes noticeable around 45 mph and intensifies the faster I go. The vibration peaks and diminishes to a consistent beat proportional to my speed. I'm sure it's there at lower speeds, but not strong enough to notice.
I recently replaced my steering links and ends in the hope of reducing steering play. I'm hoping my toe-in is just a little off.
So, I had the front end alignment done and all wheels balanced and rotated. The vibration improved, but didn't fully go away. The remaining vibration seems to be load and RPM dependent, since it diminishes when I let my foot of the gas out on the highway and intensifies when I get back on the gas. The vibration changes before the truck manages to gain or loose mph, so I don't think it's the drive shaft, which is spinning with the rear diff.
I think I may have a bad torque converter or another transmission issue. Gonna have a shop look at my C6, may do a rebuild. I have ATF dripping from the Torque converter cover, so I know I at least having sealing issues. I wanted to do the rebuild myself, but work is crazy right now and don't have time, unfortunately.
I'll check all the bolts from the bell housing, to the center carrier, to the diff yoke one more time and drive it tomorrow, see if something was just loose, but I'm starting to suspect my transmission.
