What would cause my 95 JGC to vibrate, off and on, under the drivers floor board, when it reaches above 50mph? Had the tires re-balanced yesterday! Still occuring!

Out of balance tires or a defective tire with a separated cord.

sounds like universal joint on tail shaft replace both.

The first thing to have check are the tires, the from tires since the steering is vibrating. Also take your car to have all of the tire balance, if they are out of balance it will vibrate at higher speed. Thos two are the most common issue for a vibrating steering wheel.

Tyres cause vibrations from several reasons I --out of balance== have the tyres rebalanced 2 --tread/. belt separation inside the tread ( most noticeable on the rear end as a sideways movement like a duck walking and the tail feathers move from side to side)=== maker fault 3 --running under inflated =destruction of inside walls causing unbalance 4-- wheel alignment==castor wobble 5 side bulges in the side walls == from hitting gutters or stones or pot holes Start with the balance as a lot of the faults will be apparent on the balancer.

If the tires are round once checked on a balance machine to be true. Next check the driveshaft is a weight missing, dings or bent, u-joints loose?

Then check the rear ring

get your wheels balanced at the tyre shop.adrian,,,

Tire vibration can be caused by a few things. Here's what to check:
1. Check the tire inflation. Make sure it's at the recommended pressure.
2. Have a local tire shop check your tire balance. If a tire is out of balance you'll notice the vibration at speeds above 50mph.
3.Wheel alignment. If the alignment is out, the tire will show uneven wear and can cause vibration also.
4. If a front wheel bearing is going out, it can cause vibration, but since your car is only 5-6 years old, this probably isn't the case.
Hope this helps you narrow it down.
Best of success!

tires are out of balance. have them balanced.

Most vibrations at speeds from 45mph to 55mph indicate tire balance problems. If there is no vibration outside this range have the tire balance checked. If there is a vibration that starts around 50mph and continues at higher speeds there could be a problem with the CV joint(s).

Tires would be the 1st place to check. If you seem to feel it more in your legs then it is likely the front, if felt more in seat bottom then could be a rear tire. Depending on how severe the vibration is there could be a tire developing a tread seperation and should be checked right away. A simple balance of the tire and wheel assemblies may be all that required also but when on a balancer they should be able to tell if the tire has a more severe problem.
Driveline components could cause vibrations also however if the above sounds like it fits then check that 1st.