Tires: missing wheel weight, steering wheel, mile trip

Question
I have a 90 GMC Ext Cab 1/2 Ton truck and just replaced all three u-joints in the driveshaft, it was time, but i thought they were causing a vibration in my truck.  I drove a 100 mile trip and it was vibrating really bad in the steering wheel and my whole truck at about 40-55 mi range.  when i got to my destination i noticed that i had a wheel weight missing.  the weights are as follows....FRONT: .75AL21....REAR: 1.0AL28 one of the rears is missing.....could this be my problem or it be something else?

Answer
Trey,

The missing wheel weight might be your problem.  But how do you know what is missing - afterall it's not there.  It isn't like its the same every time.  The weight is there to counterbalance the uneven weight of the tire and the rim, and who can predict what weight is needed - That's why they have machines to determine this.