The TF999 can be beefed up fairly easy with a heavy duty kit. Unless you're into auto trannies, it will cost you some to get it done at a shop.

I have a TF999 behind the AMC 360 V8 in my CJ. The only problem I had initially was burning the narrow clutch ring in the torque converter. That contaminated the tranny fluid, which screwed up the tranny. When I had the tranny rebuilt with the heavy duty parts (and the torque converter replaced), I had the operating line for the locking clutch clamped off, so it never locks now. That costs me about 10% in fuel mileage, but it'w worth it to me not to have to worry about it.