The old truck (60-F100, 292 4-speed) has developed a weird noise. At around 55 mph, I can hear & feel a rhythmic "vrum-vrum-vrum-vrum", which is more pronounced under a load, like when climbing a hill. It doesn't show up until that speed, and goes away as I approach 60.

Was thinking driveshaft needs to be balanced, or maybe just have a bad u-joint. Any thoughts?