2 months ago I went to the ford dealer to flush my transmission with amsoil synthetic on my 05 3/4 f250. Last week I noticed a oil leak between the rear engine and transmission. Anyone had similar problems?

Very smart to use Amsoil ATF in your PSD--it doesn't get any better. Examine the oil carefully, in order to determine if it's engine oil or ATF. Frequently, with oil leaks, it's hard to locate the leak from the bottom of the vehicle. They often turn out to be higher up on the engine.

Ensure the transmission pan bolts are tightened properly. If the dealer used air, it's easy to strip the threads. Since there's no torque converter drain plug on your year PSD, you don't have that to worry about. However, there may still be a rubber plug that you can remove, clean, and reinstall. Ford often has those on transmissions, even though there's no converter drain plug anymore. I've serviced many truck transmissions over the years. Ford truck transmissions are very tough--I doubt anything is seriously wrong.

Did the dealer drop/clean the pan and replace the filter?

I also have the same problem with my isuzu crosswind 4ja1. 1st mechanic changed the oil, but oil leak still exist. I transferred to other mechanic, but the oil leak became serious. He advised for an engine overhaul to treat the leak. I am bothered the engine is excellent except oil leak. Please help me

The rear main crankshaft seal is likely leaking. It's requires dropping the transmission and removing the flywheel for access. Seals go for around $9.00.