this isn't a mustang question but maybe i can get some ideas. my friend drives a '99 jeep wrangler 4.0 and when we hit about 55 mph the front of the car just shakes violently like not how it would if the tires needed balancing but more like if the motor mounts were cracked or something. any help on where to start looking would be greatly appreciated. thanks everyone!!!!

 

It probably is a tire out of balance. If it shakes around 55mph then go to 60mph turn off the engine & coast to 55. If it's still shaking then it isn't the engine

It might be a front driveshaft though

 

I know jeeps shake alot more than regular vehicles at higher speeds.

 

this has just started happening a few days ago

 

First make sure it is out of four wheel drive. The axle locking mechanism for the front axles are known for failure and could possibly stay locked if 4wd is disengaged.
Jack up the front end and check it out for loose steering parts.
The harmonic balancer on jeep 4.0's are known to go bad but the motor should shake sitting still.