So I have a 6 inch rough country lift on my truck and it shakes pretty hard when I hat about 70 mph and I was curious on what exactly I would need to fix that. Also if I would need to re gear my truck because it does have 35in tires. Any help would be great! Thanks in advance

 

You can change the gearing if you think it is helpful.

I personally would want as low a gear as possible if I drove with 35's, but not everybody thinks that way.
It depends a lot on what truck you are driving. What motor, transmission and current gears are installed.
And on your driving habits. If you drive a lot on flat highways you might be just fine but if you live in the mountains, everything is uphill and you only drive in the city, you can probably benefit from lower gears.

A person never NEEDS to regear. You can regain driveabilty with a lower gear after taller tires have been installed.

Shaking?
Could be a poor installation of the kit?
Could be a bad tire or out of balance tire -tires?

Lots of things that are hard to tell without being in the vehicle. Take it to your local tire shop for them to balance.

 

Problem #1: You will probably need to take to a shop that deals suspension lifts and driveline issues, someplace that deals with lots of 4wd and such. Given such an extreme lift the problem could be almost anything.
Problem #2: You didnt tell us what you currently have for axle ratios. A 3.73 will get the job done fine, but 4.10 would be a better option for sure.