Vehicle is my, new to me, 92 Toyota pickup 2wd with 297,000 miles. Have a vibration around 56 to 60 mph when I am giving it the gas in fifth gear. Really noticeable when going up hill at those speeds, but stops if I left off the gas. On flat land, vibration is still there, but not nearly as bad going uphill. After 60 mph it's smooth all the way to 80 mph, which is about as fast as I want to go, and there is no noticeable vibration in fifth gear from 45 to 55 mph. Searched about this problem, and what I found was that other Toyota truck owners spanning a lot of different years had the same problem around about the same speed that I am having the vibration. U-joints seemed to be the main cause, so I am going to change the front and rear u-joints. It's a two joint drive shaft. U-joints looked OK doing a visual inspection. They are not greaseable. Have some Spicer 1310 series greaseable u-joints with outside lock rings on order. Don't have a press, so I am going to try, and rent the u-joint tool from a local auto parts to get the job done. What else could cause this type of vibration? Asked the local tire shop that does Road Force tire balance, and they said the tires being out of balance would not cause the type of vibration that I am having.