When I hit about 40 kmh the truck starts to vibrate. the transmission mounts seem to be gone. however even when they were fixed the prob still exists.
can't figure the problem out at all.
anyone have any info on this?

are the tires worn out? that could cause vibration!

There are many things to consider. Tire wear and balance could cause vibration, as could worn out u-joints which you should have checked with the trans mount replacement. If none of those help, get you drive shaft balanced. I had a nagging vibration and did all the above to fix it, finally the drive shaft balance did the trick, good luck.

Worn brake rotors could be it, my honda accord had that problem until i replace the rotors.