I have a 1988 F-150. 302 AOD. When I'm going 65-70 down the highway whenever I accelerate or am loading the engine a decent amount to maintain speed there is a vibration. The vibration resembles going over rumble strips on the side of the highway. (No I'm not going over the rumble strips). I was told to check my U-Joints. I am thinking if it is a U-Joint issue it would be the one on the front of the driveshaft on the tranny side. I have replaced the rear one already on the differential side. What's your takes on this? I cannot test drive it currently and will not be able to drive it for the next week. The oil pressure line for the old gauges blew from incorrect routing, so I need to install my new ones before I drive it again.

 

Check the tires for being cupped
I agree on checking the u joints again and I would replace the front one for GPs after you replaced the rear one
When they get tight and groove the star, (needle bearings make wear marks in the star) they vibrate like you are describing
65 to 70 has me going
That is a tire imbalance vibration speed in most cases
Driveline concerns usually come in around 40 mph and get steadily worse as your speed increases
Good luck, have fun