I assume that this is a new occurence and that you have miles on it previously, in its current configuration, without issues correct?
If so, then this could be relative to a few separate causes including: condition of your trans fluid, internal trans issue (synchros, fork, blockers,), damaged motor or trans mounts, broken components or loose bolts or missing dowel pins, or a problem with the shifter itself. It's easiest to start by examining the shifter first, to insure that nothing is loose or damaged.
Keep in mind that if this is affecting a single gear only then it is most commonly associated with something internal but checking the easy stuff first won't hurt! Let me know if you have other questions and I will be happy to help.