99 xj classic with 6in lift

[shayd]

Shakes really bad when u get up to 40... cant.seem to figure out what the problem is..

 

[dc_jeep]

Take it to a tire shop and have check if the wheels are balanced

 

[TerryMason]

Does the shaking stop if you let off the gas / push the clutch in?

What type of rear driveshaft are you running (stock or CV)?

Did you change the pinion angle when you lifted it?

 

[Bounty__Hunter]

That's a ton of lift for an XJ and it takes a lot of work and experience to make it work right. The rear pinion angle needs to be spot on and varies depending on your rear driveshaft type. The front axle needs adjusted for the proper compromise between pinion angle and caster.