Vehicles and other objects are sometimes hovering above ground or partially buried below the terrain's surface.

If you've got OFP: Resistance, this is a known problem in versions 1.75 and 1.85. BIS' lead programmer, Ondrej Spanel, says this is a random error in OFP: Resistance's surface handling code. It was corrected as of the version 1.90 upgrade patch.

If you've got an earlier version of OFP and have similar symptoms, take a look here and here to see if either of those cases explains your problem.