Did you lower it with blocks or remove springs?? I am asking as if you used blocks, you don't want shorter shocks since your mounting space didn't change (just the axle moved up closer to the frame) and all that will happen is your shocks will bottom out and blow. How low did you go?? You are probably hitting the snubbers (rubber bumpers on the frame. I have 3" block on my truck and they are almost touching. I cant trim them or remove them as my tires will rub the fenders then. So look at that clearance, stiffer shocks wont help your ride either. If you removed springs, then a shorter shock should help but you still have the snubber clearance issue.
The front and rear shock are totally different also. Mine are from Monroe also even though they don't really do anything with my setup!
Here is a pic where you can see the snubbers right above the axle on the frame.
When the truck is on the ground, you cant get your finger between them and the axle.
The front, you can go with a lower shock, since the whole suspension has changed from stock specs, but on all my vehicles I have found stock shocks seem to ride smother than the lower ones (they do wear out a little faster) but the ride quality is better IMO. And the snubbers up front are probably bottoming out also on you. Some people say take them out but I prefer to cut them down to save on the metal to metal contact, esp on big bumps!
Plus all in all lowered vehicles are going to ride rougher than stock height vehicles because of the now shorter suspension travel and closer ground clearance.