If you want it to be 2WD for drag, just take the belt off. Since your front axles need to connect to some outdrives, you can't dump the front diff. a oneway should do fine (eventho it's not pulling). Compare with 4WD with oneway and see which is giving you the result.
the 2wd micro set up I saw had custom made rear foam tires which are much wider than stock.
Their front end was only made off a carbon plate on which where standing the knuckles, it was looking like a pan car front end...lot of work
Thats cool you guys are doing drag racing with micros. One chassis you may want to look into if you are not too attached to your micro RS4 is the one from BRP. Its a 2wd pan chassis with plenty of room for full size electronics so you pretty much can load it up how you want. I have seen guys mount 4 sub C cells in it and still put in the standard receiver and standard ESC. It may be the best option for you since it is very simple.
I believe the BRP SC18 is rated at either 21 or 23 mph stock. They also have a pro chassis that is plate graphite and an option to mount a big block motor on it as well ( stock chassis or pro chassis ). You can also mount HPI tires on it if you buy the mod kit for the tires.
These vehicles run very well for road courses and should run quite well for drag. You may just want to modify the steering setup alittle to use turnbuckles so its more true and buy some pan car springs to use on the front ( they also have a kit to put ball bearing supported wheels up front ).
Put it this way, if we still had micro out here, I would be running the BRP hands down. Used to put 5 laps on the micro RS4 guys.