Had the E4OD rebuilt 12mo. 10,000mi ago. Told the guy I was towing heavy and he set it up for firm shifts. Empty it was fine but towing heavy and using WOT thru 1st & 2nd up freeway ramps it was slamming into gear. I let it go till this week (not smart) then Rapid Transmission put softer springs in the shift body. Mech said it made it worse, he said it now slips between 1st & 2nd then bumps hard into 2nd, something is badly worn and needs another rebuild. I said maybe later and took it for a spin. This truck has never been able to spin the tires (has LS) going straight on clean dry asphalt, ever. From a stop I hit only 1/2 throttle, revs go to 3500 then acceleration lags a bit and revs go a little higher and then "bang" it hits 2nd and chirps the tires just like my kids Camero used to do. Sounds and feels cool but I know it's gonna tear the guts out. So I tried shifting manually 1-2-3 at WOT and it shifts perfect. Tried D and it chirps into 2nd every time. Any ideas on how to make D shift the same way as manually 1-2-3? My old C-6 trans had a vacuum modulator, does this E4OD have one or use vacuum to control shifts? Thanks for any help.