1992 GMC Suburban 4L60-E not shifting into OD, Transmission Modulator?

On the High shifting from 3rd to OD is slow. RPM rev high at before shifting into OD.

I cann’t get above 70mpg… You also loss power when passing it slows down when you put the FOOT down? 1 2 good 2-3 good

First of all, your transmission is going to be a 700R4, the 4L60E’s didn’t come out until mid 1993. There are no vacuum modulators on either the 700R4 or the 4L60E.

With that said, lets get to your complaint.

Several things can cause the symptoms you describe. TV (Throttle Valve) cable being mis-adjusted, too tight in your case. Governor sticking or “lazy”, also, a leaking 2/4 band servo. I would start with the cable, then go to the governor next.