You need to find out if it is just too short or if the slip yoke travel is not long enough. If it is not close to fully compressing, then you can lengthen it or add a spacer:
- http://www.4crawler.com/4x4/ForSale/DriveshaftSpacer.shtml

If it is compressing well and still pulling apart a longer slip yoke is probably needed. You can get long travel slip yoke kits or you can make a new shaft out of square tubing or get a new shaft built.