I recently washed my truck and sprayed down the engin with a low pressure and since then, while driving the truck starts to shake and vibrates in the lower rpms. Once I accelerate the vibrations subsides. Any Idea what could be wrong?

 

Better start blowing out the spark plug wells. Sounds like some dampness got in. Put some dielectric grease in the boots before reinstalling them and that helps to waterproof them.

 

Model year and engine?

On the ones with the conventional distributor setup - what you describe is typical of moisture under the cap. If it's a hot, dry day - just pop the cap off and leave it for a couple of hours, otherwise you may wish to consider drying the insides with a rag and perhaps spritzing some WD-40 around to displace the moisture.

Of course, there may be other issues causing this - just the quick 'n' easy approach first.