I agree with Lugnutz on the rear mounts. Also if you have the single center front mount you have to retain the wide rear mounts to prevent the engine front rolling over. I don't think the mounts are your problem. If I am understanding you correctly you are noticing this at 60 mph in both 4th and 5th gear which set engine at to different RPMs. This tells me the vibration is being caused by something behind the transmission. What happens if you get you truck up to 70 mph place trans in neutral let engine idle down then let truck slow down through vibration range. If vibration is still there it is in the drive line not engine. The odds of it being in the transmission are slim so that leaves the Ujoints, drive shaft, rear end, or wheels/tires.
Start by making sure that you don't have a bent wheel or a broken belt in a tire, also the balance of the tires. If they are fine then I might consider taking it to a different shop to have the drive shaft/ ujoints checked out. Just my thioughts.
