My 4 speed stick rattles around when I'm in 3rd or 4th at speed. What should I look for to stop it from jumping around.

Thanks in Advance,
Lori

Find out where the vibration is coming from and fix it.

Take a look at the transmission mount.On these older cars most are pretty worn out. Also check that the crossmember bolts are tight.

it could be loose bushings on the ends of the shifter arms to the rods

The shifter body may need a rebuild or it's coming loose too possibly. Buddy in high school cured his with a rubber band pulling on it from the side.

If you've never had the driveshaft balanced I'm going to guess that this is the source of your vibration. If it has been neglected for a long time, it would be a good idea to have the driveshaft balanced and have new u-joints installed. The driveshaft may also be slightly bent which is another item that can be addressed when it is being rebalanced. A worn transmission mount can accentuate any existing driveshaft imbalance.

If the shifter itself is buzzing due to the mechanism being worn out and loose, it would also be a good idea to have the shifter rebuilt or replaced. Often times, the only real problem with a shifter is that the shifter rod bushings need to be replaced. Greasing the main shifter mechanism would be a good idea as well.