I need help. My 82 gmc 4x4 front tires{38.5" swampers} start to shake an jump around really bad at around 45mph.I checked to see if things are broke but found nothing.What else can it be?  

Are your tires balanced? Are your wheels bent? Have a tire shop check them out and if they try to balance the have them use powder weight. They should be able to tell you if tires or rims are good. Are shocks good?  

Sounds like classic death-wobble to me.  

might need dual sterring stabilizers on it with 38's i think you would burn a stock or single one up  

I have had this happen to me on my old suburban w/35's.  If the rear of your truck is lower than the front, the caster gets off causing the death wobble.  All the steering stabilizers in the world will NOT make this go away.  They will help, but not make it go away.  
Other things to look for; worn/loose tie rods, drag link, ball joints, king pins (on D60 or D70), loose steering box (loose on frame and/or internally loose), shackle bushings shot, bent wheels, tires out of round, you get the picture...
If this stuff is all ok, I would then look at putting caster wedges in under the front springs.  If you really want to know what the caster is- take it to an alignment shop, they can tell you.  I know this costs money, but is it worth driving around like it is now; worrying about loosing it & running into something?  
Hopefully this helps smooth things out.  

Sounds like a tire issue to me, we had a 84 3/4 ton chevy with a set of those 37in "ebay" military goodyears  the back end jumped at 50 or so, tried to have them balanced, the right rear called for 2.5lbs on the spin balancer at the shop!!!