If you can swing it, go for the center diff. It will be easier on the front and rear diffs, there will be less rotating mass AND less overall weight, and it's just so much more awesome

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As for locking the tranny into 2nd, I haven't the slightest idea. I have never opened up that tranny before (only my Savage 25 tranny that came with my Savage).
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz