4 wheel dive PLEASE HELP

I have a 1994 S-10 4 wheel drive ls. I recently had the transmission rebuilt by a reputable shade tree mechanic (if there is such a thing). Anway when pushing the 4 wheel drive buttons, It shifts (or gears down)to 4 wheel low of hi, but the 4 wheel drive doesn't work?? Is there a reason it would be gearing down but not engaging the 4 wheel drive. Any info would be appreciated.
Thanks

Also, it's a manual transmission

this may help on my old 95 blazer there was a unit the passenger side front ( at the bottom) of the engine compartment that has a vacuum line check to see if the vac line is disconnected and on the transfer there are vac lines also check to see if they are disconnected

thanks for the advice, I'll give it a look