2003 2500 HD Durimax/allison Transmission rear seal leaking

[Leefy]

Just had my truck in for a transmission flush and new synthetic oil put in. has 95,000 miles. Has not shown any signs of leaking on the garage floor until after I had this done. When parked over night it drips about the size of a $.50 piece. It is coming out of the rubber area around where the spline for the drive shaft slides into the transmission.

I am assuming that I must have a rear seal out. I don't plan to fix myself but wondering if this is a major repair or not.

Any input would be appreciated. Hate to be shocked at the repair shop. Don't know if the synthetic had anything to do with the leak. Can tell that the trans shifts smoother and runs cooler. I have already lost to the low cold transmission area on the dip stick so probably lost a couple of quarts in 2-300 miles so must be coming out pretty good.

Leefy