Toyota Repair: 1997 T100 6 cyl, 2WD, ext cab, driveline, steering wheel

Question
I am getting a terrible vibration that appears to be somewhere in the drivetrain when driving between 50 and 60 MPH. It does not happen all the time--I will drive several days and then it appears again, but recently it is present most of the time. Tires were recently rebalanced and alligned.  

Answer
Have you checked the driveline. If you can crawl under your truck while parked, grab the driveline with both hands and see if there is any free play. There shouldn't be any. Have you isolated it by reving the engine up to the same RPMS as when you are driving at 50-60mph (roughly 2200 rpms) and make sure the engine does not have a broken motor or transmission mount? Lastly, if while driving and the drivetrain vibrates, do you feel it in the steering wheel? This will isolate the front end from the rear end. Good luck