assuming gear diffs rather than Ball or torsen:

Easy and permanent: Take out your diff, open it up and JB Weld it into one solid lump. put the diff back in.

Slightly harder, but temporary: take out your Diff, open it up and jam as much Silly Putty in the gears as you can fit. put the diff back in.

harder, but less messy: take out your diff, open it up. take one of the spider gears off and then put it back slightly out of line with the other 2/3 gears - this should cause the diff to bind up and not move. put the diff back in.(works on some but not others)

expensive, but easy: see if they make a "spool" for your model buy it, and fit it.

what model is it anyway?