Have you taken your truck to an alignment shop yet? It sounds to me like you have a negative caster angle.
Many times when suspension lifts are added, spring perches changed, or shackle lengths changed it is likely that your caster angle has also change as a result.
I built a 88' k5 blazer with a skyjacker 8" lift, NO steering stabilizer and the the truck drove like a cadillac at 75 plus MPH...
First thing I would do is take it to an alignment shop and have them tell you what the caster and toe angles look like. If your CASTER angle is negative than BINGO you found your issue.
Way to fix...
1. Add longer front shackles. This will roll your axle downward on the pinion side causing the caster angle to become more positive
2. Shim. You can add a shim or wedge to both sides of the spring perches where your leaf springs mount in order to gap the rear side of each spring perch which will again drive the caster angle to a more positive position.
3. Cut and turn your steering knuckles. This is a very technical procedure and if you do not have adequate fabrication experience I would not attempt this. When I lift a vehicle I will typically roll the axles to set the pinion angles for both front and rear drivelines. When this is done the front axle will be rolled in such a fashion that the caster angle becomes quite negative. By cutting the axle housing you can roll the steering knuckle to it's factory caster alignment position and blam-o you're cruisen..
This video will give you a basic idea of what I'm taking about:
http://cutandturn.com/
Your Caster angle is your stability angle. If it's negative you wobble if it's positive you're golden.. There are other factors that could cause a wobble that have been mentioned but if everything checks out this is where I would look next...
