The transmission in my truck went out about a month ago. After exploring many options my dad told me he would buyone from a wrecked car and have a friend come help me put it in the truck…needless to say, after 12 hours of work we got the transmission in but to no avail. I have a 2006 gmc sierra 2X4, standard cab, reg. bed, with a 4L60E trans, I was sent a trans out of a 2005 chevy tahoe…All the wires and plugs fit, all the components looked the same and we didn’t have any problems installing tge trans. But when we started the truck and put it in to gear nothing happened; its as if the gears don’t engage at all. I am wondering if it could be an electrical or computer issue, or if something may be wrong with the trans that I put in. Please please help…!! 
I will assume you have checked that the fluid level is up to the ‘cold full’ mark after running the engine and shifting through the gears; that the torque converter is bolted to the flex plate; and that the rear drive shaft is not turning when the transmission is in gear.
When you removed the oil pan during the condition inspection was the residual oil red and not smelling burnt? Was there any debris in the pan? Did you change the filter and use a new filter gasket?
This is unlikely to be an electrical or ECM problem as you will get Drive and Reverse even without any electrical input (you would be in ‘limp home’ mode in Drive without solenoid drive power or computer input.) Next step would be to check the line pressure at the available tap to see if the pump is working.
At this point getting a replacement transmission (it should be done under warranty from the truck recycler establishment) and doing the R&R again is going to be the most cost effective avenue.
Hope this helps.
I’m not being facetious but it is full of fluid isn’t it? Next step is scan the truck for codes.
Just how certain are you that this transmission you were sent is not someone’s cast-off that is nothing more than a rebuildable core?
Now, when you have the tranny rebuilt, you can provide the shop with TWO cores!