Got a question I have a 98 ford f150 4.2 I run a Junk Removal company and haul trailers with this truck all day bought it maybe a year ago truck has been serviced before I bought it and had new tranny and fluid currently has bright red fluid.

Problem: driving down the road at exactly 30-40mph truck shakes violently but after I pass 40mph it goes away.

New:
U Joints
Balljoints
Wheel bearings
Truck is equipped with a transmission cooler.
New Filter
New fluid.

My "mechanic" said it may be the torque converter but its not slipping and still has good power just the horrible shake is my problem. My sway bar links are broke but i dont think that would cause it? Was a good truck before the shaking continued & im stumped any help is appreciated thanks.

 

could very well be torque converter shutter at that speed.

 

When I had my transmission rebuilt I was told of a product called Lubegard. Did some research & decided to add it to the fluid. It supposed to add a better degree of lubricity
to the ATF & claims to help cut down heat. I can't say one way or another if it does because I haven't put it to the test by towing yet.
Although they do offer a additive specifically for torque chatter. If your not crazy on having the trans rebuilt now, granted it's a band aid fix it just may get you by until you do
rebuild it.

 

Could very well be your trailer. How's it drive without the trailer? Can you post a pic with the truck and trailer on flat ground. I can already see something odd with your trailer. Tongue is down in front and your axles are down in the back. Are both the trailer axles still in proper alignment? How are the trailer tires? A load leveling hitch wouldn't hurt regardless of your problem.

EDIT: Are both of your axles on the same leaf spring?

 

Truck runs fine and has no problem fluid is good & i added the shutter fix stuff still no resolution. Noise still happens when trailer is off or on. Im clueless at this point as its my daily driver also I just had the tires balanced on the truck & still no positive results

 

My friend owns a trans shop and he puts Lubegard in every trans he builds. It's good stuff. Been using it for years.

EDIT: For what you are trying to accomplish, you want the GREEN bottle.

 

X2. i would look into it

 

First off, don't disregard what anyone is telling you by what I write. Everyone is helping you with what they know and none of us has seen your truck.

I hesitate to get into suspension discussions because people seldom listen.

You're wrong about your sway bar, antiroll bar, stabilizer bar or whatever you want to call it. Particularly if you also have a rear bar and the front links are broken. Our ladder frames are strong but they are not stiff. They're trucks even if most people use them as daily drivers now. They're still built to handle torsional loading like a work truck should be able to handle. They're not rock crawlers but they're more than a road vehicle. Most of what you will find on sway bars will relate to sway and roll related to turning with a road car but on a truck they're about torsional stability also. With your broken links you compromised your trucks torsional stability. Those links are part of your trucks torsional stability. This effects your weight transfer during acceleration, deceleration, braking, etc. This causes vibrations.

You've said the problem only shows up between 30 and 40 MPH. That issue can only exist for one reason which is vibration and vibration harmonics. It doesn't matter if it's your transmission, the frame or your trailer, that's the issue you're dealing with. If it wasn't you would be experiencing the problem all the time. Your broken sway bar links can definitely be causing this. It is a bit odd that the trailer makes no difference to the 30 to 40 MPH. That has to do with axle torque and how much of it gets transferred to the frame and suspension during acceleration, deceleration, braking, etc.

If you understand differential equations and trig identities along with knowing your frame and suspension dynamics we could prove this but $20 on new links would be faster and should be done anyway if you're towing.

I still think it has more to do with your trailer. Even if the problem source is the transmission, the cause I believe is that trailer. See how the trailer side supports don't go all the way down the trailer? Your trailer is not strong and has little stifness. (They're not the same.)

 

You did not say if you checked your tires for balance or maybe an issue with one or more of your tires

 

Google Death Wobble. Its mainly a Solid front axle thing but its always fixed by the Sway bar replacement.