Unless they changed the speedometer set up from the 88 trucks to the 90/91 trucks it should be a gear driven speedometer. Its not an electronic speedometer that can just be calibrated. And speedometer gears are transmission specific, so your going to have to find ones for your truck. I was looking on 4X4Wire and here are the relevant part numbers.
MD705462 - Gear, Speedometer Drive (N=22): For 4.625 gearing and 33" tires
MD705461 - Gear, Speedometer Drive (N=21): For 4.625 gearing and 35" tires
They don't make one for 34" tires, so either way your speedometer will still be off. I don't know if they even still carry them, but you will have to order the speedometer gears from the Mitsubishi dealership. I have used Larry Jay Mitsubishi before for my Raider/Montero, there may be one closer to you though. If there aren't any shops in your area that will do it, here is the write up:
http://www.4x4wire.com/mitsubishi-tech/speedometer-gear-adjustment-for-big-tires-on-a-montero
You may luck out that they don't make them anymore. Get proof from the dealership they don't make the speedometer gears anymore and give it to your lawyer/bring it to court. Or you could argue they don't make one for the size tires you have.
And I don't know how the officer found out that your speedometer was off, but if you get pulled over again I wouldn't mention it/use it as an excuse.
The only other way to "fix" your problem is to put the 30x9.5s (still bigger than stock 225/70s) back on the truck before you go to court or talk to the assistant DA. Which technically will bring the truck back close to the correct speedometer reading. After court is over, put the Swampers back on and hope you don't get pulled over.
Best of luck.