im going to buy ajustable lowering shackles 3-5 iches and one store told me i would have to buy shimes to re aline my pinion angles is that true

 

Not required but definitely recommended. Without the pinion angle shims you will most likely get driveline vibration due to the altered angle from the rear end to the tranny.
You won't be able to put adjustable shackles on the Dodge ( if it is a Ram 1500 ) due to the way DCX mounts the leaf spring ( shackle mount is above the leaf spring mount point ). You will physically run out of travel between the mounting eyes. The GMC on the other hand has the shackle mount below the spring. Making the shackle longer will lower the vehicle fine.

 

I dropped my truck 4" in the rear (99 Silverado Z71), and so far I've been lucky as far as vibration goes. I bought 4* pinion angle shims just in case though....so far so good, but I'm hanging on to them just in case a driveline vibration develops. I've had the truck up around 85 mph since the drop and it's been smooth as far as I can tell.

You could always do the drop first and test drive it to see if the shims are necessary...they're cheap enough to buy as some insurance just in case.