As a general rule I am hesitant to open to 4x4's with mud tires because of rollover concerns, but I am going to defer to SCCA rules in this case
http://cms.scca.com/documents/Rally%20Rules/2009rxr.pdf6.1 ELIGIBLE VEHICLES
A RallyCross event is open to any fixed-roof production based vehicle (including
convertibles with a factory hardtop attached, targa-types with factory panel in place,
t-tops with factory panels in place) that can pass safety inspection. If the Event
Chairman after consultation with the
Event Safety Steward determines at his discretion
that a vehicle cannot safely negotiate the course, it may be excluded.
2. Tires must be DOT approved. Snow tires are allowed. No studded tires
are permitted
unless ice or snow are present. Studded tires may not be
homemade using bolts or screws. Only street-legal studs are allowed.
Tires may not interfere with any parts of the car [fenders, fender liners,
suspension, etc].
“Aggressive” tires are prohibited. A tire will be considered an “aggressive
tire” if it meets any of the following criteria:
a. It does not carry a DOT approval.
b. It is marked “For competition only”, “Not for street use” or similar
verbiage.
c. It is modified or altered from its original tread pattern in any way.
d. Tires must be the original size plus/minus 20mm cross section and
5% aspect ratio.