I installed a used transmission in it and now it.

I installed a used transmission in it and now it wont go into any gear I checked the fluid and its good and I took the top line off the trany cooler and it has good flow changed the nuetral switch. I cant figure out whats going on I got the trany out of a 1999 suburban and put it into a 2002 avalanche everyone I talked to said they are interchangable I put the bellhousing and the tourqe converter off the old 02 trany on the 99 befor I put it in so any help would be great thanks