Not sure where you'd have a longer leaf spring stack made up, but adjustable vertical links are available from the likes of Wolfitt racing -
http://www.wolfitt.com/wolfitt_produ...htm#Adjustable rear vertical link kit
It would be a lot simpler and cheaper just to change the drive shafts though.
Oh, have you checked to see if there's a lowering block fitted? If there is then taking that out might correct the issue.