Grab it and shake it up and down and side to side. If there is no play, remove it and flex the u joints by hand. If they are bad, you should have play in the joint or spots that are not smooth when rotated. As old as these are, you may just replace them while it is apart. The grease in them has probably long dried up Just mark your yoke, and joints, so you put them back in the same orientation as they were before.
Not knowing any history about the 1028, how long have you had it. Have you been driving the truck. Is it a new vibration or has it just started to vibrate? Are you in 4 wheel drive or two high?
sneekyeye is on the right track. Check the pinion nuts, the drive shafts are famous for being altered by military forklifts. Be VERY careful and chock front wheels and lift rear end center differential with floor jack, place jack stands on axle, and put in neutral and grab driveshaft by hand and spin slowly and observe the shaft for being true, straight.
If the u joints do not have grease fittings, well they are overdo for replacement anyway.
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