Hey 47, I'm not an A.D. guy and definately not a guru but I can point you in the right direction. I've been looking for an overdrive for a couple of years. I'm not exactly a mechanic so I didn't really understand what they were, I just knew I liked the results. Now I know that it is an auxilary 2-speed transmission that will go somewhere between your transmission and the rear axle. I have one that I just bought recently from Gear Vendors. It bolts right on to my sm420. I will be able to overdrive 2nd, 3rd, and 4th. You can set it so that you can engage it manually or set it to shift for you electronically between 4th and 4th over or between 3rd and 3rd over. 4th over will give me the equivilant of 3:08 on the highway with my original 3:90 posi. Best yet, I get to keep my 4spd with granny and 3:90 rear which can be so useful for my region and activities. I see you're from B.C. so I imagine there are some hills where you live. I do have to have the driveline shortened and I have to have my rear torsion bar crossmember fabricated to allow clearance. The later probably won't be a problem for you unless you've upgraded to torsion bar IFS. Warning, the unit and the process is not cheap.
www.gearvendors.com They can answer your questions or maybe can tell you where you can get one that will match your application. I hope this helps.
