You gotta get a tune for your new stall. A good tuner will be able to make your new conveter lock up like it it should. It's not a tranny problem at all. Your truck probably won't pull as hard on the big end as it used to because your new converter is a little less efficient on top end than your stock converter was. I lost 21 hp on the dyno locked vs unlocked. But what you gain on the bottom end is well worth a little loss on top.
08 ECSB 5.3 2WD TSP 224/224-581/581 112 LSA, TSP 7.4 Pushrods, Comp 918's, LS7 lifters, LS2 Valley Cover, Ported Throttle Body, MSD 8.5 wires, Yank SSTB 4000 Stall, Yukon 4:56 Gears, Stock Posi Unit, Pacesetter LT's with ORY, Made 323 RWHP, 318 RWTQ With Converter Locked Rolling on 20's, Made 302 Unlocked. Tuned by BlueCat w HP Tuners