I have heard about the 'rebadging' option............been told the dirt bikers do it so that they can put fat 4WD tyres & rims on their rigs.......however they tell me its a pain in the arse costly exercise with engineering certificates & more.
Engineering certificates of compliance are only required such as welding modifications, that are subject as to changing the structural integrity,as to to mechanical variants.This falls into AUS/NZ ADR regulations.
To have your vans GVM de-rated is simply a change to a lower GVM; meaning your van is after approval rated 2.8 GVM this then also solves your modification of de-weighting. to satisfy your insurance company that the lesser tire is safe for the lower weight.
NOTE: in doing this modification in effect you cut your load carrying capacity in half to a bout 800 Kg max.
The problem is not the tire so much, it's the type you have as to Summer Tire Highway use. but equally as to a Manual-Transmission; the first and reverse are too higher geared; for a slower take-off on say damp grass, or a hard sub-clay packed dirt road with a marbling like fine dust road based lose surface. or say sand, where you just bog down.
Automatics are designed for a slower speed take off, your option of either fully 5-speed automatic, or clutchless six speed Sprint-shift but again the final axle ratio on your van is not set for off road surfaces, mind you that was never a priority with the Tins or the current NCV models, this has been since rectified, with the New 2nd generation NCV 2009 August Euro 5 month delay for Aus and I won't go into that one. Of which has a brand new Manual transmission with first gear being set for a slow take off and reverse same deal [two variant options for a manual Transmission] Designed for building sites, or off road use where it is an issue getting into the sites under various weather type conditions, where inclines are varying. the ABS system can't adequately handle the extreme conditions. hence ASR-ABS manual cut off.
If you looked at Australian states as where the sprinters are prominently sold S/A W/A NSW and upper Queensland, used sprint-shift as to back roads non-sealed full automatics were proven to be a total failure in Australia as to the GVM ratings from 2.8T to 616T variants. 4x4 Sprinters were limited to the public but based on outback serviceability being not available, was the main reason for holding back on these, variants.
Bearing in mind I know how many automatics failed in both Mercedes and VW as to the nag1 4-speed-5-speed was taken directly from the sedan and was not rated to be over 3.2 GVM NCV 2006 was a redesigned H/D transmission which had little problems if used correctly.
Finding the right tire is a hard call as to the above..
Richard