Engine/Transmission (1998.5 - 2002) Transmission Upgrade?

[BHawkes]

I have a '99 extra cab, auto transmission, long bed with 93K miles. I have noticed that when I don't drive my truck for a few days, when I go to back it out of my garage, one the transmission is placed in reverce, the truck doesn't move for several seconds. I was told by someone that my torque converter was leaking fluid internally and just needed time to refill. Having said that, I have been looking at upgraded transmisisons as of late, and would love some advice from those of you who have experience with this issue. I'm wanting to also enhance the performance of the engine, and don't want the tranmission to be the weak point in the truck. I have looked at a couple of manufacturers of upgraded transmissions, and want some input. Help! Thanks.