I don't know why but when I go above 50 mph my truck starts shaking but seems to go away at higher speeds.
This is likely a tire "out of balance" issue. Most vehicles have a certain speed where an out-of-balance condition from a rotating component (wheel, driveshaft, etc.) will resonate, or "echo" the out of balance condition - causing vibration, and even a drumming sound in the vehicle. This vibration goes with the shaking. Once you are past the resonance speed, the echo effect goes away.
Have your tires balance-checked, and re-balanced if necessary. If not the tire/wheel assemblies, the driveshaft is the next-likely source of this problem. I would also recommend having the car's shaking/vibrating symptoms inspected by a mobile, professional mechanic, such as one from YourMechanic, to diagnose this problem, get an accurate assessment of damage and cost estimate for repairs.
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