My 89' Blazer with a 4" suspension and 3" body lift, running 35" tires sometimes shifts out of third gear between 40 and 55 mph only after I hit a bump. It will be running at about 2000 rpm and the rpms will drop to about 1600rpm and I am driving the truck in drive, not over drive. The tranny was rebuilt a year ago and it didn't do that prior to the rebuild. I went back to the shop that rebuilt the tranny and they said it was because of the lift and tires. That I find hard to believe ! Any ideas, I am kind of sick of listening to it down shift every time I hit a bump.