The shims are for setting the the crankshaft "end play" and they MUST be reinstalled correctly for the rear main bearing thrust bearing to have the proper clearance.
If the shims are inversely left out when the flex plate (flywheel) is bolted back up to the crankshaft, it can lock up the crankshaft on this engine due to lack of proper clearance.
Crankshaft End Play
0.080-0.280 mm / 0.003-0.011 in.
This is covered in the FSM in Section 9
http://www.colorado4wheel.com/manuals/Jeep/KJ/
_________________
Supporting Vendor and Moderator of LOST05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount98 Dodge Cummins 24 Valve