Thought I would update a problem I had with my 2000 LS Blazer 4X4.To go back to the problem,I would shift from 2hi to 4hi and it would not shift,it would neutral the TC out usually,sometimes it would go back to 2hi.It never locked in the front driveline.I always did have 4lo,no problem,but no 4hi.I checked out as much as I could,actuater under battery,all vacuum lines running from and to TC and actuater,all plugin connections,including TCCM under kick panel,checking for corrosion,and anything that didnt look right.I went ahead and replaced selector switch on dash for the hell of it,which really wasnt necessary,but it made me feel better!I couldnt find anything that stood out,so I took it to the chevy dealership that is close to me.They had it about 3 hrs and diagnosed it as being the TCCM.They replaced the TCCM and it works as good as new.It costs me close to 400.00 but well worth it,afterall.........its a CHEVY 4X4 BLAZER!!!!! Hope this helps someone out! RobD