1998 4.2 Automatic
 

If im going down the road accelerating at a pretty slow speed and i dont change my foot pressure on the pedal. It seems like right when it shifts into 4th the engine races pretty loud then kicks in. The truck kinda lunges when it kicks in there going down the road. I thought maybe I didnt have enough vaccum pressure or something going down the road and might have a link. What do yall think?

How high does the engine race. A couple hundred rpm spike is normal because the converter clutch releases for the shift.
Alan

well I dont have a rpm gauge on the XL model. But it sounds like it revs up pretty good.

Overdrive band is probably scorched and slow to engage.