Shaking problem

The truck drives fine until it gets up to about 40. Then once it hits 40 in 3rd gear, it will start vibrating and shaking. It almost feels as if it's coming from the transmission? Or possibly the motor? I don't know, and haven't been able to pin-point the problem. But once I get out of 3rd, and shift into 4th and get up to about 50, the vibration goes away. And that's the only time that it does the vibration and shaking. Any ideas/answers? I could really use the help.

Any chance it could be bad rotors or a balancing issue and not an engine/trans problem?

I honestly have no idea. Like I said, I've been completely unable to pin-point the problem.

Try the easy thing first, balance the tires

Okay. Will do. Hopefully that will knock out the shaking, but who knows. Lol.

i agree with hooligan start with the easy and work towards the hard... yeah it may seem like your putting alot of money in to it... but look on the bright side when you finally figure out what's causing the problem all the stuff that you have already changed won't need changing for a while.

could be a mess of things. Balljoints, TREs, unbalanced tires, broken belts in the tires, worn out bushings, etc. Get under there with a light and start trying to move ****. Also check the tires for any unnatural wear or bulges that might be visible.

I havent seen alignment yet...my 1987 is doing the exact same thing and im 99% positive it needs tires balanced front end alignment possibly brakes. Could be one or all three but when im done it wont do that anymore. Let us know what your issue is/was

also, don't forget to look at the drive shaft. weight mighta fell off, u-joint froze up, got a nasty dent from high centering on a rock while 4 wheeling...

Had the same issue of zones where it would shake hard. I could tell it was driveline though. Felt like it was right under me AND the shifter had a lot of vibes to it.

I was expecting to check the drive shaft and find all kinds of play. Turns out I had none. I actually parked the truck for a while thinking it may be the rear end or the transmission output.

Finally one day recently I got mad and took the drive shaft out. Turns out on side of the joints is a little stiffer to pivot than the other. So they are not coming out but rather getting stiff and causing the vibration.

Gotta get somebody to get them changed out for me because I fear bending the ears on this thing. I just don't have the correct tools for it.

Can't wait to have my old truck back. I might fix the dang thing up now, as it is of the age now it would be kinda cool again

If it turns out not to be mechanical, what is the alternator output measuring ?

If it is not putting out the required voltage things can get weird.

Jack up each front tire, try to wiggle the tire to see if it moves. Your just checking the bearings. You can also spin the tire and see if it makes any noise, like a grinding sound. Unbalanced tires could cause it, but it usually progresses with speed. If you don't think it is front end related and instead engine related, do you have any check engine codes?