When they changed your transmission they unbolted the rear transmission mount and cross member allowing the transmission to hang down unsuported for esier acsess to the bell housing bolts this I have learened twice causes the rear main seal to leack also if you pull the engine and transmission and dont block the transmission so it is leval with the engine when you seperate them it will cause the seal to leak. Replacing the seal is no big deal about $10 and over 40 hours of labour, Your leak may be small enough that the stop oil leack additives that rejuvinate seals and are only to be used for a hundred miles before changing the oil do work some times you should disconectt your crankcase vent hose from the valve cover leaving the hole in the valve cover open to vent into the engine bay this will reduse the crank case pressure stopping the leak from getting bigger till you try the oil additave or and good luck with this one get the transmission shop to pay well over a grand to fix there screw up. I left a transmission hanging unsuported for a month the seal leaked a quart a day but with the vent tube off it barely dripped I left one hang for ten miniutes to get at the bell housing bolts eiser it sprung a small leak that stop leak fixed I bought two other cars that developed leaks right after they had used transmissions installed and leaked I got them for a song and with the vent tube disconnected and oil additive for a hundred miles they never leaked again