do both. change the ujoints too. its most likely the trans mount though. in 89 i took mine to three diff shops, they all said it was the ujoints and bolts for the ujoints. all three times i changed both and didnt fix it. thought for sure it was me f'in up the job. replaced the mount just out of spite and it rode smooth as silk all the way to the buried speedo. ujoints are cheap though, and since your down there anyway.
Don't overtighten those u-joint bolt, as some have reported some vibration (and ruined joints) from overtightened nuts. Torque specs are 8-15 ft/lbs, which is just a bit snug with a wrench by hand.
