It all depends on how the lift was achieved. Without knowing or seeing the truck in question, only assumptions can be made.
Regardless of the method of lift, one can only hope a drop tracbar bracket, drop pitman arm, and adjustable tracbar were used. Those will need to come out. OEM tracbar bracket and OEM pitman arm will need to go back on. The tracbar might be able to be reused. Socks will likely need to be replaced with shorter ones.
If it is a spring lift, then you will need to remove the leaf pack and replace with stock and then use whichever 'leveling kit' you want to go with.
If it is a hanger/shackle lift, then a stock version is required and then the method of 'leveling kit', or use a 2" hanger/shackle kit.
If blocks were used to lift the rear, then those need to be replaced with stock.
Way too many variables to tell exactly what it needed.
Many manufacturers only utlize a track bar drop and drop pitman arm on a 5" lift or taller given the stock mounting locations are ok up to about 4.5" of lift and allow for max travel but can cause bind over 4" if not engineered properly. Depending on the manufacturer, you're looking at:
Add-a-Pack/Front Block (Could be a spacer instead)
Track Bar Drop (possibly)
Exended Length Track bar in Stock location (If Icon or Carli)
Drop Ptiman Arm (possibly)
Longer Sway Bar End Links or Sway Bar Drop Bracket
Radius Arm Drop Bracket
Brake lin/ABS Extension Brackets
Rear Blocks and U-Bolts/Rear replacement Spring pack
Taller Rear Shocks
E-Brake line relocation Bracket
Carier Bearing Drop Bracket
As stated by beta racer, there are many variable as no one builds the same kits and lift is achieved by many different means. It may be better to have a professional handle it if you're new to susepnsion work.