If you have unlevel ground you can make x braces like this... we bolted through the two 4x4s so it was strong and put two on every 8' section. We also put those 24" mailbox spikes that you can get at home depot onto the end of the 4x4s on the deeper side so they would dig in and set.
bolting each section together while the rail is floating and then having 3-4 people stand it up into place.. when it finally tips up and then falls, the spikes will set them self and its pretty stable...
this was the first slider we built and it was really heavy... but it was portable and worked with the unlevel ground.. it also took qite the beating in the intracoastal from the big boats but was there over a year before we took it out..