Check your engine and tranny mounts again and see if your driveshaft is bent. Was it shaking before you put the new motor and tranny in too?

silverado shakes
 

i will check them tomorrow. yes it was shaking before the new motor and tranny . but it does feel like a bent rim or unbalanced tire . i already eliminated that, i changed wheels and tires and it still shook at the same speed 60 to 70

silverado shakes
 

well i checked my drive shaft and found a bad center u joint . i could feel the play in it so im doing them all and the carrier. they are not that old . the motor and tranny mounts were tight. cant get the carrier off so ill take it to work tomorrow.

silverado shakes
 

did my u joints and carrier but still shakes. dont know what else to replace
rack and pinion maybe ? this shake is like a splinter , its driving me crazy!! soon i'll have a new truck (with a shake).

Weak frame flexing under the stress. Try tightening all your cab bolts.

silverado shakes
 

well i camped out under my truck and pretty much went over every bolt,nut ,brace, mount etc. found left rear shock bushing wore out and loose . also got my wife to shimmy the steering wheel back and forth while i watched the rack and pinion and the rack moved back and forth about a 1/4 inch total. i dont know if its supposed to do that? i have a couple of friends at work that have silverados so i'll check theirs out tomorrow.

Did you put her rear on jack stands and put in gear and see if you could see any shaking??

silverado shakes
 

i did that also on monday . started out slow to ckeck the tires and rims then took it up to 65 and it ran fine. had my wife do it first so i could see then did it myself for the feel

Visually check the leaf spring bushings. Where they bolt up. Might as well replace them if your gonna take apart to see. But those make a good difference in ride. Bushings are pretty cheap. Bout 20 for a set.

I think you should check the wheel alignments of your vehicle. The problem may be there.