I wouldn't discard your axle setup either Steve. What's the total ratio of your axles? To go with a motor at each wheel in this trk you'd "have to" power the front axle too or the rear tandem drives would push you straight!
I've been using this wdc twin motor setup for more then ten yrs with great success and add on the 4:1 Carson (pmd rip off) behind a Tamiya 3spd & it's a slow moving stump puller. This setup is so slow in my trks that I rarely use 1st gear. I think your tires are less than twice the diameter of mine?
pmd bought out wdc's remaining supply of twin motor cases, so you're stuck at buying it at his ripp off price! Will the twin motor from Tmaxx, is it? fit the the Tamiya some how? The wdc is not a direct bolt on either, but an adapter is not difficult to make. I'm pretty Willy has some of these Carson 4:1's at a fraction of the price & Willy fabbed up some aluminum cases too like the originals. I use the original case (plastic) in my 6x4 trks & mount the 4:1 directly to the trans. You've seen a pic before, haven't you?
I'd try that out before doing anything drastic! 1st gear should give you the precision driving you want loaded & 2nd or 3rd to haul azz back to your reserve pile.
I think the back of your pickup will be too small for your 990 to maneuver, you'll need a track hoe instead.
