[ddsmith]

If it's popping out of gear it's a tranny problem. Disconnect the linkage and remove the side plate. You will probably find some gear teeth or bearings laying in the bottom.

I disagree.
Jewels,
I had the same problem you describe on my Corvette. Well almost the same problem. My car has a M-21 Muncie 4 speed and the vibration was very noticeable through the Hurst floor shifter. It would pop out of fourth gear when driving on the highway forcing me to maintain slight pressure on the shift lever to prevent the shift into neutral. My problem was traced down to a very wore pilot bearing. It allowed the front shaft of the transmission to move a small amount up, down, and side to side. This action created the vibration and the shift lever moving into neutral. When you drop the transmission. Inspect the pilot bearing and I'll bet it is wore far beyond what is acceptable. If it is wore bad use a bolt slightly larger than the inner diameter of the pilot bearing and long enough to make contact with the crankshaft when you thread it into the pilot bearing. Grind the end of the bolt to a taper that is small enough to enter the hole of the pilot bearing. Start threading the bolt into the pilot bearing and it will cut it's own threads as you go. When the bolt contacts the crankshaft the pilot bearing will remove itself. Good luck. Hey, pilot bearings cost about $3 to $4. Now that is what I call a cheap fix. Good luck.