It seems i have developed a vibration in 4th and 5th gear but only after driving 20 miles or so... could a u joint cause this? the vibration is slight..any suggestions?

first off the drive line will rotate at the same rate at a given speed , regardless what gear your in. It spins according to the gear ratio in the Diff.

Yes a bad u-joint could be the reason for vibration. It would show up at a higher speed.

thanks for the info..what joints would you suggest i check first? rear half?

It seems i have developed a vibration in 4th and 5th gear but only after driving 20 miles or so... could a u joint cause this? the vibration is slight..any suggestions?

Does it start vibrating at any speed after 20 miles and does it continue. A duff u joint would normally make itself known as soon as you started driving. Have you checked/changed the diff/tranny oil?

no i havent changed either.... it probably needs to be done... is it 80w90..

What I'm getting at is that you may have a bearing related problem in either the diff or tranny caused by low oil levels. 80w90 in the diff but I prefer 75/80 semi synthetic in the tranny although some might disagree.