Yup, the '99 Durango box is a direct swap as everyone else has indicated - except for having to space it away from the unibody a bit.
How do you tell if your's has spacers? Simple, use a small punch thru the end of the box's case (there's a small hole around the edge just for this) and tap out the large snap ring that retains the dome shaped end cap. Any good rebuilder shop should have no problem doing this for you before you purchase a box from 'em. Turn the input shaft to press out the end cap and then look at the inside surface of the end cap. If you see 3 raised ledges around the inside surface - you've got spacers. If flat all around, then no spacers.
Removal of the spacers: Either swap out the end cap with one that doesn't have the ledges, or grind 'em off. The other spacer is at the opposite end of the piston chamber. You'll either need to already know how to rebuild the box, or have a rebuilder remove the piston and shaft to get to the spacer behind the piston - pretty complicated if you don't know what you're doing. That spacer looks like a large "rounded triangle" snap ring. Discard that spacer and re-assemble the box. This will give you the maximum "lock-to-lock" throw that the Durango box can have.
Mine works great!
