1993 Ford Ranger: Vibration above 65 mph

1993 Ford Ranger, 4 cylinder, 147,000 miles. I recently changed my shock absorbers on my truck. Shortly after that I developed a violent shake at above 65 mph. Had a complete front end inspection by 2 different mechanics. Found drivers side wheel had left-right play movement. Replacement of the center link fixed that problem. I've had 2 alignments performed by both mechanics, had the wheel bearings re-packed, and all tires rebalanced and rotated. Both mechanics found that ball joints and tie-rod ends are good and in working order. Even after 2 alignments, I still have a slight pull to the right and there is some play in the steering along with the vibration at high speed. Vibration starts at same speed whether in 4th or 5th gear, so I don't think it's a motor mount problem. The only thing I can think it might be is the Steering Gear Box, but I can't afford $400+ repair only to find out that isn't the problem. Could the shocks also cause a problem like this? They do seem pretty stiff after 2 months of use. Shocks are "Grabriel Ultra". Thanks for your time.

Matt