My car is shuddering really bad at around 45-55mph. Below that, it cruises around perfectly fine. Above 65mph it seems to improve as well. It has the original springs front and back. I recently replaced the front shocks with some Monroe gas shocks. The rears do have aftermarket air shocks put in by the previous owner. I let some air out of the air shocks and didn't feel any change. Will replacing these air shocks with gas solve my problem? Thanks.

Do you feel the shudder in the wheel? the floor? When you apply the brakes? Could be numerous things. Could be a tire/wheel vibration, bent wheel, driveshaft outa balance, bad u-joints...
Could be a out of round drum if it happens when the brakes are applied... Little more specific maybe, other wise those are my thoughts off the top of my head.
Phil

I would guess a tire out of balance. I've had them sometimes seem to "smooth out" after reaching a higher speed, I've also had them continually get worse.

Can you tell if its coming from the front or back?

I doubt changing the shocks will have any effect unless it's only doing it after hitting bumps or something. If it's constant, I don't think the shocks are the problem.

Its not when I'm braking, the car brakes work great actually. I feel the shudder in the wheel,seats, the whole interior shakes. Hard to tell if its coming from the back or front.

I'd start by getting the tires balanced to eliminate that variable.

If you feel it in the steering wheel, it's probably the front, but it doesn't hurt to balance all of them.

How good are your tires? Maybe one is starting to separate or bulge? If so, you'll catch that as soon as you put it on a spin-balancer.

If you feel it in the steering wheel, it's most likely in front.

If you feel it in your butt, it's probably in back, or the driveshaft.

I'll pull the front tires off and have those balanced first. I looked at my wheels (Cragar SS') and I don't see the usual lead balance weights on them I'm used to seeing so I'm guessing they were never balanced. Thanks for the help.