why would my front end stakes at 75-80mph in one but not 70mph in under?

check front end components and tires if eveythings seems ok then I would balance and rotate tires

Get up to 75 , then gently apply the brakes while maintaining 75 , if it stops shaking , start with the struts.

seek insp from alignment shop and they will see loose parts if any are worn

A vehicle's front end may shake for numerous reasons and it is important to set up an appointment to fix your car. It is possible that parts may require replacement and maintenance may be needed. A factory trained technician at Barbera's Autoland can test drive your vehicle and try to verify your complaint. Please call 215-333-3700 to set up an appointment today to correct your vehicle and receive a diagnosis quote. We look forward to hearing from you. Thank you, Jessie Barbera's Autoland Chrysler Dodge Jeep Ram