Shmoo-
There are dozens of thorough and thoughtful threads on this topic on this forum. Check out the 'search' button at the top right of the forum home page. There are also tecnical articles on the transmission upgrade.
Basically they'll tell you that your problem isn't necessarily the rear differential, its the gear ratio in your transmission and rear differential. So, you've got some choices, improve the ratio at the rear diff or improve it in the tranny.
First option: Change the ring and pinion gears in your existing axel to something a little more friendly to modern speeds. I think Patrick Dykes of Patrick's Trucks sells the ring and pinion plus the kit to install it (shims, bearings, bushings, gaskets...) All you'll need on top of that is a tools. There are several other vendors who sell the stuff, but it lately they've all been on back order. Total cost, probably between 1,000+ (parts are $600+)
Second option: Get a rear differential with the gear ratio you'd like to have AND also replace the torque tube with a driveshaft.
Third option: Do something about your transmission to get better gear ratios there. There are bolt in kits to swap your existing SM420 to a Saginaw 4-speed, which has a much more useable gear ratio. To use the Saginaw and keep your torque=tube you need the rear housing off a 3 speed, a shifter (the saginw is NOT a toploader like the SM-420), and the Saginaw itself, a very common tranny.
Fourth option: A combination of both- Upgrade your transmission to a modern 5-speed and replace the rear differential and drive shaft. I paid $100 for a rear differential at a junk yard and spent $200 going through the brakes and differential. The drive shaft cost me $25 and I spent $100 on new u-joints, balance, etc... The T-5 tranny cost me $100 and I'll probably spend another $150 on a 'short thow shifter' for it.
The SM420 you've got is probably 7:1 in first gear, combine that with 4.11:1 in the rear end and you turn the engine a lot to get the tires to go around once, and max out your speed in that gear pretty quick. That was great for pulling stumps but for driving in todays traffic...
My t-5's first gear is 3.76, combined with a rear diff ratio of 3.72:1 I'll be going 30 in first before I have to shift and in overdrive will be crusing at 75 at the start of my engine's powerband with plenty throttle left to pass cars or speed up. Plus, I'll get much better gas milage.
