Specs:

82 CJ5
2.5L
Manual 4spd trans

I get to around 30MPH and the whole rig starts vibrating. Then around 40-45 it stops. Doesn't vibrate in any other MPH range. Doesn't matter which gear I'm in whether 3rd or 4th gear. I'm thinking its the suspension (really old rancho shocks). Is there any other cause? HELP!!! I need to know before I spend some of my limited funds on expensive parts.  

Bias ply tires? Been sitting around a few days? Does it get less vibration after driving a few miles?

These would be indications of flat spotted tires causing your vibration.  

I drive it everyday... Checked the tires a couple of days ago and they're okay. It does it all the time. I was told it could be the U-joints. Is this true?  

If a tire is out of balance it will vibrate around 30-35 mph and again around 65 mph. all other mphs you dont notice it. at least thats what i learned working for Goodyear.  

Well it's time for new rubber anyways. Any suggestions for a 31" daily driver. I've got some bfg all terrain on there now and they've been good to me. I only go on trail rides anyways. (for now) I have my mudders on separate rims.