Sounds like the speed sensor in your diff died, it's a simple fix.
"They say ignorance is bliss..... I don't know what the Fu*k that means but I'm pretty happy". Ricky Trailer Park Boys.
Current Mods....X-Brace, Stromung Cat Back Exhaust, 17/225/45 M3 Replicas, HotBits Adjustable Coil Overs, 40% LSD 3:91 Diff, E30 Gearbox, Boosted.