90 GMC 4x4 VIOLENT front end shake at 45-60mph

I just acquired a 1990 GMC 1500 4x4 extended cab long box a week or so ago. The previous owner said it had a VERY violent shake in the front end that would start at 45mph and stop at 65mph. His mechanic looked
at the front end and couldnt find anything wrong... He was unable to try to balance the tires as he didnt have a balancer big enough for the 16 inch rims... but gave him a set of rims and used tires to try on the front.
He didnt get a chance to swap them before he got a new truck and parked this one. Before i drove the truck i swapped the fronts. I didnt have the shudder on the way home. I had the old fronts balanced and put them back on. I drove it for a few days with no problems other than a slight vibration from the rear(havnent balanced those yet, will do in a day or so).
Yesterday I drove at highways speed for about 20 minutes, I decelerated off the off ramp and then back up to highway, i hit 45 and there it went BANG BANG BANG BANG... darn near shook the doors off.
I parked, went to the place i was going, got in and drove off. Same road, same speed. No issue.
It seems to be somewhat intermittant. I'm thinking it could be the stearing box as the stearing is loose. The ball joints, etc look good. I just redid the brakes, oil, transmission fluid, plugs, etc...
The only other thing i can think of is a belt slipping or blister in the tires.
Any ideas?
Thanks JOsh
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Its neat to watch from the outside too ;)
What size tires? Do you have a steering stabilizer?
Cheers

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On 18 May 2007 09:15:05 -0700, snipped-for-privacy@gmail.com wrote:

You kinda have the signs of a caster death wooble. This happens when the combo of caster, toe-in and camber creates a dynmically unstable front end that given the chance it can shake REALLY bad. Loose front end components can agrevate it. Is this truck lift anf have oversized tires or is it 100% stock? ----------------- TheSnoMan.com
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I had a problem like that on my 95 suburban. turns out that the control arm bushings needed to be replaced.

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