Exactly what I was going to suggest. If you are very careful, you can actually upshift and downshift without using the clutch if you match RPMs as suggested above. I also agree that it sounds like synchros to me. Here is my version of the RPM explanation. This will be better on a less busy road. Drive along at about 40 MPH in 4th gear. Then do a normal downshift to third, but do it a little slower than normal and increase the RPMs by about 800 or so. That will allow the speed of the gears in third to more closely match the engine's RPM, and make the shift smoother. If there is an improvement if the downshift, my guess is that you have bad synchros. The transmission was supposedly rebuilt??? There is a big difference between what I consider a tranny rebuilt to be, and what others do. The previous owner could have replaced a few bearings and seals and gaskets and called it good.
