What Scott and blt2ski said.
Be more specific with your issue, aside from height (wouldn't mess with height by modifying suspension unless you can do a simple spring under axle flip and have the clearance at the wheels to do it.
Smooth the ride? Is it riding to "stiff"? I.E. The springs are actually overkill for the weight (an uncommon TT occurrence) or is the suspension seeing plenty of flex but it's bouncy?
If the springs are suited for the weight then shocks would help.
If the springs are too stiff, then lowering tire pressure (assuming additional tire capacity would help).
Although a simple issue, there are about 3 potential causes of an "unsmooth" ride.
Torsion axle, IMO is not necessarily the answer unless you size it for the actual weight.
I.E., a 5k torsion axle on a 3k trailer will still ride maybe rougher than you're wanting.
My boat trailer is an extreme example, twin torsion axles, rides great with 4000lb + boat on it. Like a rock empty.
My 7klb enclosed trailer twin torsion axles, about 3500lbs empty, rides bad (hard and bouncy) empty. Smooths out once my total weight is about 5klbs or more.
