The 1992 frame mounted hitch will fit on your 1990 -- but the big difference is in the springs. 1991 was the first year they used progressive rate springs (get stiffer with more weight) capable of handling anything over 200 lbs. at the rear bumper... that's why the manual says the towing limit is 2,000 lbs. asssuming 10% at the hitch. It's not that your engine/vehicle can't handle more, it's that your rear end could bottom out from the weight. I'm sure this will cause debate, but that's the reason you don't see a Class III available for your year. You can put Gen II springs on your truck or buy aftermarket ones from Old Man EMU or Valley Springs to bring everything up to spec for towing 3500lbs. There are articles about this in the tech section. Of course, this still would only allow 350 lbs. tongue weight including your receiver carrier. Long story short, I tried to go the same route as you with a 330lb motorcycle, and it still didn't work out right. Seemed unstable. I just bought a trailer.

Good Luck!
Bryan
Dallas, TX