tires
 

hi i have 33X12.50 15 on my truck and my lift is at an unknown height. it has plenty clearance, that is not the issue. however at 35 - 40 mph the truck shakes violently. the person i purchased the truck from claimed it to be the tires size as the issue. is this true? if so what tire size should i seek i can post pictures. thank you for your time,

Tire size would not cause shake... bad tires, out of balance tires, ball joints, idle arm, u joints... lots of things can cause shake and or vibrations but NOT tire size...

Most like the term you need to be googling is "death wobble".

ifs or leafs?

so if your wheels and tires are the same as the one in your avatar it looks like you have 12" wide rims which is a bid wide IMO.
Now if your tires have not been rotated properly that can definitely cause issues if one is worn more than others.
IN addition to what has been mentioned in post #2 the following can contribute to Death wobble: bad front end alignment, bad tie rods, bad wheel bearings etc can also contribute, tires not at the same pressure
(you can upload pics by clicking on the little square with the mtns that shows up in the options bar when you go to post, that will allow you to upload pics from your phone or PC)

looks like you were are infantry- Leg or airborne?

yall are awesome! i greatly appreciate all the help. as for my tires being too wide, what problems can this create? id like to slim them down. ill just have to invest in more wheels as well. ill have to get out and take more pictures

oh and i dont believe it to be death wobble as its more of and up and down shake. and it doesnt effect my steering wheel

Go pro a wheel. Watch video in slow motion.
Find a friend in your area with small tires, swap and check.

Might well be an out of round tire?

If it still does it, replace all ball joints and steering joints. Get it aligned after.

Maybe there is slime in the tires or the spacers don't fit properly and have the wheels mounting off center.

I am going to use some.term loosely here, you've been warned..

The king pin center line (this is an imaginary line, there is no King pin) should intersect the road at the center of the tire contact patch (tire center line). It is the primary purpose of the wheel rim offset. If this is not the case you can get uneven wear on the tire, due to scrubbing, which can cause imbalances and thus vibration.

You can inspect for this sort of damage either by eye, if it is severe enough, or a tread guage.

One solution is the tire beads which are intended to redistribute and balance the tire every time they are rotated above a certain speed (centripetal force).

The next option is to exercise your "life time balance and rotation clause". Some shops will only.do this every s thousand miles, for example Walmart tire says something along the lines of every 6000 miles.

The ultimate option is to stay within the design limts and match your tire width and offset with the steering (King pin) angle.

In another note.

If your tread is sized such that you pick up the local gravel or rocks you need to get in there with a hook and pick them out.

Ps/Pet-peeve.. A wheel is a tire and rim! You need new rims to go with your slimer tire.

I know you have gotten a lot of good advice. One thing I have not seen (and makes me curious due to your steering wheel not shaking) is your drive shaft. Jump under the truck and shake the driveshaft on both ends. A lot of times those U-joints don't get greased and go bad over time. I had this issue with the runner I bought. The U-joints were causing the driveshaft to shake until it shook the nut to my transfer case bearing shaft loose. This caused a violent vibration. I was able to swap out the joints and tighten up the bearing to specs. All is good now. Sometimes the best thing to do is just get under it and grab and shake.

thank you everyone! i got the truck in and out of the shop for 240$ i found a good deal on some tires online and got the truck shop .9 miles away from me to put them on, (they came to get the truck) i let them know of the misfiring issue, they ran a diagnostic, and they found a leak in the intake tube. the truck fired right up and ran great! no more bouncing around
thank you all, the help was greatly appreciated and helped me ask what needed to be asked at the truck shop.