Derestrict a Sprinter

Yeah, 90 is about tops for a 311 and would be a much better option for more pleasant motorway driving, especially when trying to overtake the idiots who sit in the middle lane at 70.
3.5t vans aren't limited to 60mph though, maybe you're getting confused with dual carriageways? Limits for 3.5t vans are; Motorway 70mph/Dual carriageway 60mph/Single carriageway 50mph.

I'll have a hunt around near the fusebox and see if I can see anything. I've read a post on a US forum that says if you turn the key to the start position (apparently the starter won't turn once the engine is running) it disables some limiters. Yet to try that though.