Glat to hear you're enjoying the truck... I have one myself and its a blast to drive, very tough construction too.
For the receiver pack I am using IB Intellec 1400 cells... If you can find one of those get them, IB cells are well known in the electric world for their quality.
For the failsafe, I'm using the one from duratrax. Very compact and so far no issues at all.

For the transmitter, I've tried 2400 nimh AA packs (from sony) but ended up hating them (they now lose the entire charge after 24 hours so I have to keep recharging them)... I have a set of Sanyo Nicd cells that are 5 years old and that just won't quit. Just 700mAh but they keep their charge for more than a few days. I'm definitely getting more nicd's for my transmitters instead of Nimh