1998 Chevy Tahoe lurching on downshift

Recently ready to sell 98 Tahoe. Started to lurch/jerk on downshift under heaving braking to a stop (like clutch not disengaging). Original tranny so decided to fix before selling. Tranny shop determined it needed to be rebuilt for $2k to solve problem (not what I wanted to do but did it). They had problem after rebuild getting into gear but replaced brake switch. I took it home and found that I have the same problem. Obviously they part swapped without diagnosing the problem. Question: if tranny has truly been rebuilt, what else can cause this problem? ECU, MAF sensor, another brake related switch? Tranny seems confused about braking/speed. Dont' want to take it back to the shop with some research first.