Vibration with hubs locked

1985 F150 4x4 with 4.9 and NP 435 manual transmission.  When I lock the hubs, I get a vibration that is proportional in speed with the rolling of the tires.  No movement, no vibration.  Even vibrates when still in 2wd with hubs locked.  Checked the U-joints and all related mounts, and can't find a problem.  It's so bad, I've only gone about 8-10 mph in 4wd.  Anyone have any ideas?

nilknarf007
1985 F150
4.9 IL6
NP435
4x4

I suspect you have a bad u-joint in that front axle.  So I'd lock one hub at a time and see which one causes the vibration.  If one does cause a vibration I'd lock the other one in, by itself, and engage the transfer case to prove that everything else is smooth.  If that is smooth you'll know where your problem lies.

Don't lock the axle on dry pavement?

There should be a sticker on top of the driver's sun visor explaining this and why it isn't a full-time 4wd system.

Totally off-topic for this thread, and sorry I can't help with your issue, but how do you like the Clifford setup?

Thanks Gary.  I gave that a shot, and I got the vibration with both sides.  Keep in mind though, I'm really going slow.  The drive shaft to the front end is in really good shape.  Not going fast enough for a u-joint problem to rear its head unless something was missing from it.  I'm afraid if I go faster than idling in 1st gear (actually 2nd), something will fly apart.

As far as switching the truck into 4wd, I'm familiar with how to do that.  Plus, there isn't any dry pavement for miles.  We're in the middle of a good snow.

I did notice, however, that my 4wd shift lever doesn't have the positive "detents" that it used to.  They're still there, but not as pronounced as it used to be.

Most of this era TC levers use a block of rubber to push the stick against the detents.
You might find the rubber has hardened with age.

Edit: is the pivot pin rusted solid or packed with mud?

How is the pinion slop and backlash in the front axle?

Sometimes the lube in the front diff never gets changed and the gears and bearings eat themselves.

ArdWrknTrk, the rubber you mentioned has seen better days.  I'll have to look into once I can spend some time underneath while dry.

There is no slop that I can feel.  There is no play front to back, side to side, or rotationally.  Oh, and I can rotate the front drive shaft with my hand while in neutral and unlocked.  It's not easy, but I can rotate it.

I changed the front diff fluid when I bought the truck a little over 2 years ago.

Okay. We can eliminate bad mesh and the pinion getting sucked into the carrier as causes for your jerky front axle.

And you say it doesn't change going straight ahead or turning?

Nope.  Stays the same.  But I haven't driven it over about 10 MPH in 4wd due to the vibration.  Don't know if it goes away at speed, but I doubt it based on what I'm feeling.

I'm beginning to wonder if it might be in the transfer case.

Have you tried both high and low?
Is the front output slip or fixed yoke?

Can't get it to go into low, but I can't get under it enough to see if there is an obvious problem with that.  It's a fixed yoke.  I guess I need to find a lift so I can see under it.

Try dropping the front driveshaft and driving it with the hubs locked.  If there's no vibration then youi know all the axle U-joints are good.

Another diagnostic to try could be to leave the hubs unlocked and put the transfer case in 4WD (with the driveshaft in).  Not that I know what that would tell you, but it MIGHT be interesting.

But another thing you said would concern me.  you said that it wasn't easy to turn the front driveshaft.  It should be pretty easy.  So I'd want to figure out where the drag was coming from.  (If it's really cold out there could be a fair amount of drag from the ring gear in the gear lube.  But otherwise nothing should be very hard to turn.)

Your shift stick is probably rusted up to where it can't move sideways enough to get into the neutral gate.

I'm going to suggest PB Blaster or the corrosion buster of your choice and just working it back & forth until it starts to free up, then red 'n tacky grease or something that won't wash off

R&R to clean it on the bench can take a bunch of time and you have to be careful to reassemble it correctly.

Bob has some good points, but if you can't get under to lube up the linkage on the side of the gearbox I don't expect you can pull the driveshaft.

If you have access to a lift put it in gear while up in the air so you can get a better look at what's bent or binding

Please report back with what you find

Nothing Special wrote

Try dropping the front driveshaft and driving it with the hubs locked.  If there's no vibration then youi know all the axle U-joints are good.

Another diagnostic to try could be to leave the hubs unlocked and put the transfer case in 4WD (with the driveshaft in).  Not that I know what that would tell you, but it MIGHT be interesting.

But another thing you said would concern me.  you said that it wasn't easy to turn the front driveshaft.  It should be pretty easy.  So I'd want to figure out where the drag was coming from.  (If it's really cold out there could be a fair amount of drag from the ring gear in the gear lube.  But otherwise nothing should be very hard to turn.)

I do like this if he can drop the front shaft as it would tell where the issue is.
Dave ----

I've been driving in front wheel drive for a few weeks now.
The one observation I have is that universal joints are not CV axles.

They're always going to bind and jerk the truck around especially in low speed turns.
And that is more acute depending on how much traction you actually have, given your tires tread and width.

this is why drive shafts need to be installed with both universals set at the same angles and be as true as possible in the most normal driving position. fully loaded or empty is the extreme of the range.

When you turn the wheels the universal at the hub is necessarily going to be at a more acute angle than the fixed angle between the center case and the control arm or beam

Now here's a brilliant idea for Chad/little beefy.

You want a retrofit stuff to these trucks?
Come up with a CV axle retrofit. 💡

Years ago I met and 'wheeled with a guy who had a '70s vintage Toyota Land Cruiser.  He was really excited because he had just installed a kit that swapped single Cardan joints in to replace the factory CV joints in his front axle!