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.