Definitely go with a lock up kit. The one I will be installing is from Bowler Transmissions. The thing I like about the Bowler kit is that lock up occurs 8 seconds after it shifts into OD. This gives you time to get up to freeway speed and then lock up occurs. It also comes with a brake light switch, so if you tap the brake, lock up disengages and you have another 8 seconds before lock up. I believe the cost is $179 for the kit.
