Why does truck shake when breaking but dont when breaking on a curve

Could be a bad tire ( or bent rim Have balance checked), ball joint, idler arm, wheel bearing, steering tie rod end or defective rack and pinion unit. Lift vehicle about 1 inch off ground ( do each side independently) Use a pry bar and pry from ground up and down on wheel. Much play? bad ball joint. With both hands, try to move wheel side to side. Much play? could be a tie rod end or bad wheel bearing. Visually check play of steering arm into rack and pinion unit, more than about a half inch play, bad rack and pinion unit. Try to move back and forth with hands top and bottom. Much play? Bad ball joint or wheel bearing, verify visually that it is ball joint moving, else it is the wheel bearing. Manually check for play in each joint in steering control rods from steering knuckle all the way through each rod where there is a joint. Replace the one you find excessive play in. Check mounting studs for sway bar for loose fit, deteriorated rubber bushings or a crack / break in the sway bar. Replace if needed. That's about all I can think of unless it only happens when applying brakes, in which case it could be a warped rotor.

Replace the ball joints and/or rod ends.

Very little info for a fix. Tie rod and upper ball joints do not make a front end shake. Lower ball joints wear out before the uppers do.
Front end shakes at what speed, if you have a broken belt in one of your front tires you will notice it more at low speeds. Mud or snow stuck in wheel will make them vibrate at speed. Lug nuts tight since front end work? You need to give more info. Switch front tires with back tires. If shake is in steering wheel it more than likely is front end but if you feel it in your seat it is more than likely in the rear of the car.

To remove the lower control arm raise and support your car with jack stands.Next you want to remove the wheel that you are replacing the control arm on.Remove the ball joint nut and tie rod nut.Tap the steering knuckle to break the tie rod end loose from the knuckle DO NOT HIT THE TIE ROD SO HARD YOU DAMAGE THE THREADS.If you do you will also be changing a tie rod end.Tap on the steering knuckle until your ball joint stud comes loose.Remove the 2 bolts that mount the control arm to the frame.To install the new one just go in reverse.

you should keep all four tires on the ground !!!! LOL ! your rotors or brake pads are bad. do you feel it shake in the steering wheel ? Check these three items and im sure it will fix.......1) jack front car up and try to move front tires in all directions, does it feel loose at all? if yes check tie rod ends, upper and lower ball joints. 2) check brake pads for wear, heat from pads warp the rotors and cause the break pedal and steering to shake or vibrate. 3) If nothing can be found wrong with the tie rods or ball joints i recommend changing the rotors and brake pads, a pretty cheap fix.

Several things could cause this. Look at the inner tie rod end bushings. They are behind the engine block and are easy to replace. You can check this by holding onto the tie rods and seeing if they move back and forth. If not, the go to the tire. Holding the tire at 9 and 3 (clock times) try to shake it back and forth. If there is a lot of play (movement) This could point to either your lower ball joint/control arm assembly, or your outer tie rod ends. Both of these fixes are pretty easy if you know how to work on cars at all, but you will need to get it alligned after you are done doing the work. Also, if the lower ball joint assembly is out, a lot to times you will feel a clunk in the break pedal like your breaks are releasing at diffrent times. (you can also hear a clunk.) I have more ideas, but this is a good place to start because the parts aren't too expensive. If you need advice on replacing these parts, I can give you a hand with that too. Good luck.

Have the front end checked asap, it could be a worn tie rod end, or ball joint, if either one of these break, you will loose control of the steering.
Other possibilities is
cv joints
Wheel bearings.

Best bet is to have an alignment shop do it since you will need special tools to seperate the ball joint. They may find the knuckle ball joints need replacement also. Since you will need an alignment anyways its best to have a pro do this. I have a 98 expedition with 100,000 plus miles and both the lower ball joints have been replaced recently.

I would have that front end checked as soon as possible. It could be anything, ball joints, tie rod ends, cv joints, could be the rack and pinion steering. If any one of these things break, you will loose your steering.

inspect ball joints and tie rod ends