Hard-shift at 4th gear

When my ‘95 Lumina van (with 3.8 engine and 4-speed automatic) shifts into 4th gear, I feel a jolt or clunk when accelerating on the level or uphill. When it shifts going downhill I don’t feel this jolt. Then when going uphill and slowing down, I feel a little jolt when it goes back into 3rd. Also, now and then, when in 4th gear, about 45 or 50 mph or so, I’ll feel sort of a "rumble" or vibration for a few seconds (maybe in the front halfshafts?) I don’t have any problem in the other gears. If a CV joint is going bad, would there be these jolts and vibrations in only one gear? Any ideas? Thanks.

I'd almost suspect a TCC with harsh and delayed release...they will sometimes chatter in and out of lock rapidly at the speed you describe when starting to go bad...

I don't think it would be in the axles...

You should probably check the motor mounts and roll restrictors too..