Swivel Hub Preload / Death Wobble Help!

Hi guys, got a question about setting my swivel pin preload, i'm trying to identify the source of my 'death wobble' (Steering wheel wobbles/shakes a lot during deceleration in a straight line).
My swivel hub has no play, but is not smooth from lock to lock. it's very free in parts and a little stiffer in others; has anyone come accross this before? The top pin has no shims at all so i'm not sure how i'd increase the preload anyway.
Any help would be greatly appreciated!

have you tried it it with steering rods removed,having no shims would nt be a good sign , what age is the truck,pre 94 or later

Like James says, track rod ends disconnected, also are you trying it with the swivel seal still fitted? Worth pulling the top pin and have a good look at the bearing and the outer race, the race should be totally smooth, doesn't mean it will be ok if it looks ok but if there is any damage you need to replace them and start again.

Wheel bearings can give rise to wooble too.....

Apologies for the late reply, i've been having computer issues.
Many thanks for your replies but I tightened up one of the front wheel bearings (very slight play) and the wobble has been significantly reduced - enough for me to ignore it for now anyway!

I've said this before, but it might be worth saying again in this context. The biggest improvement I got in my handling was by replacing the swivel bearings top and bottom. The originals weren't loose, but as you can see in the picture below, they were marked. This is one of the outer races:

Well worth doing in my opinion. Yes, you could feel some unevenness as you turned the hubs from lock to lock by hand. It looked like the rollers and the race had worn in the places where they most frequently bore weight, i.e in the straight ahead position.