Tranny issues. Any Ideas??? 85 burb

Hello, Just purchase my first suburban. It is an C10 4x4 with a 6" lift. The issue is: It shifts hard and at high rpms out of 1st. into 2nd. All other gears shift fine. Any ideas what it might be? Thank you for taking the time. Al

sounds like a vacuum problem, i had it happen few times on my other gm trannys.
Go under vehicle and look at passenger side, towards back, along edge of tranny. There should be a vacuum line there. See if its off or crack around where it goes on.

Mine did it when it got yanked off by something

good luck