I have a 1968 1300 pick up truck and I'm loosing interest. Because I over paid. Considering the amount of work I had and cont to do on it in order to put it on the road for three trips to the home depot. Now the latest when I got her on the freeway I notice that the whole thing shakes at above 45mph. And if you stop pushing on the gas it mellows out and it goes away even more if you get it out of gear. Any sugestions where to go other than to the next owner.

On mine it is the slop in the transmission yoke going into the transmission. I changed the U-joints and had the driveshaft balanced and it still did it. I could minimize it by the angle I put the yoke back into the transmission. Kind of hit or miss. I also put a couple of hose clamps on the front of the driveshaft and rotated them until it allmost never happened. Was good for about a year but I notice it is starting to do it exiting from the freeway. I suspect to properly fix it I will need the change the bushing in the transmission.

I don't know what to say about this problem, but my truck does it too. It's caused by the firing pulses of the engine that get amplified by the axle wrap of the leaf springs. It hits a resonant frequency above 45 mph and just feeds on itself. New shocks and bushings could help this problem, but it may never go away completely. Oversize/unbalanced tires make it worse yet. Make sure the driveshaft is balanced, and the tranny mount is good.

mine did it quite a bit when i had my tires balanced at places that aren't necessarily as 'qualified' as others... i started going to discount tire to have my tires balanced and haven't had an issue since

Always have your tires spin balanced not bubble (static) balanced.

when ever you remove your driveshaft make alignment marks on them before removing so you can get them reinstalled same as before removal. Some reason balanced drivelines on these little trucks really matter.