If your truck shakes at 45 mph, most likely the wheels are out of balance. Other causes may be a bent rim or bad belt in the tire. Most of the time, shaking between 40 and 65 is due to balance. Have them check the balance in each tire first so you know where the problem is.
Tires and/or wheels out of balance?
Wheels are probably out of balance or you may have a bad tire.
a 1500lb truck moving at 45 mph
A shake that is that speed sensitive is usually a bent rim, or bad tire.
Could be a problem with the lockup converter (overdrive).
It is 45 mph.
It is possible that the shaking in a Pontiac Montana at 45 MPH is the result of failing bearings or tie rod ends.
problem with your transmission in 5th gear
its not spossed too...you may need to balance your tires
I have a 1999 jeep cherokee, and had this same problem. After time, when i would hit a bump in the road my whole car would seize up and buck uncontrollably. It ended up needing two new wheel barrings. They were not easy to replace!!
its the pitmin arm or track bar i have seen it over an over again
check and replace tires and tie rod ends as neccassary