There is no restriction on what you chose to drive. You will be placed in category according to your vehicles engine. You can even drive 2wd or 4wd, (Shabab did the recent Jhal rally in 4X2)
There are several areas in the track with very deep ruts, where low body clearance can become big liability, causing so much drag. Many of these areas are made worse, specially near all the check points (approx. 10 of them during full race), where one has to get the race card stamped. So you stop, and than you try to push off as quickly as possible, digging further track and causing more ruts.
Also most of these WRX imprezzas or Evo would have smaller engine (2.0 litre), with big turbos, so rev too little to take off and you bog down, rev too hard and you dig yourself in like crazy. Getting the balance right not the easiest, specially when your body under parts are dragging in sand. Off-course if one can lift these vehicles up and put bigger tyres (like Mini's in Dakar), they would be in serious contention.