The O.P. is referring to a travel trailer ( I believe). That being the case there's a couple of things you may want to try.
First make sure your tires have max pressure ( mine have 65 PSI). Because the trailer is resting on the tires you don't want them soft (rubber bounces).
Next there is a trick some use: they first level the trailer left to right and then front to back. Then people will lower the front hitch jack a little and snug up the rear stabilizers and finally raise the front hitch jack. This puts some pressure on the rear stabilizers.
Chocking your wheels is always a good thing because it not only reduces some shaking it also is a safety feature to prevent the trailer from moving once you disconnect your tow vehicle.
I have found that if you mount a pair of stabilizers just in front of the axle (on the frame) this also adds to stability.
Finally if all else fails then you may be a candidate for something like these:
or:
http://www.steadyfast.com/
or:
http://www.jtrv.com/our_products.html
Your choice.
Happy trails,