Chevrolet Repair: chevy front end vibration, front end vibration, wheel bearings

Question
I have a 1990 Chevy 1500 4x4 350 engine automatic tran. with about 230k miles.
During the past month the truck has developed a vibration in the front that can be felt in the steering wheel only @ 55—60 mph. In 2 wheel drive. It is worse in 4-wheel drive.

We thought it may be the cv joints, we replaced with a new shaft assembly on both sides, we still have the vibration.  We then removed the front gear case and looked for a problem. None was found, we did replace bearings and seals, the vibration is still there.
I have my old wheel bearings (replaced about 12 month ago) and may remove the new ones and try the originals to see if the problem is in the new wheel bearings.

We installed a new crate engine ,it has about 4,000 miles on it. The first 2,000 miles were trouble free, then the vibration started. Tires are near new and recently balanced.

Have you encountered problems like this and can you point me in the right direction to resolve this problem?
I like this old truck. It still looks new and I would like for it to drive like new once again.
Thanks for your help: George Stone

Answer
Hi George,
Get it going down the road at the speed the vibration happens, and shift the transmission to neutral, and let off the gas. See if the vibration is wheel/axle related, or engine RPM related.
If the vibration stays the same, look at the rear driveshaft "U"joints for slack, and check the driveshaft for dings.
You could even jack the rear end up, and sit it on jack stands, and see if the vibration happens with just the rear wheels turning.

If it is RPM related, remove the fan belt, and run it at that RPM for a few seconds to eliminate belt driven accessories.

Then there is the front air dam if you have one. Any chance it, or maybe a fender liner or flap is flopping around in the wind?
Van