There is no automatic transmission drain plug. To drain the fluid you have to drop the pan. Be sure to replace the filter at the same time, it is located under the pan.
You have to know whats wrong with the transmission before you can fix it.
Buy a new transmission oil pan gasket before your transmission blows from leaking liquid.
It is a completely different transmission than what was used with the diesel before.
Do a VIN search and the VIN will tell you the original options.
If you have an automatic transmission it is on top of he steering column just before it goes through the firewall. If you have a manual transmission it is on the transmission.
If it's leaking from the pan gasket, just replace the pan gasket. If it's leaking between the engine and transmission, you have probably developed a leaking front seal. There isn't really much you can do for that except to replace the seal. Drop the trans, slide the torque converter out, replace the seal and put the trans back. Many transmission shops will tell you that once you have a front seal go it won't be long before the transmission fails. It's all a crap-shoot, so take your chances. If the leak isn't too bad you can try some Trans-X transmission leak stopper. It might give you a few more months service out of the transmission, but you'll probably need some major work soon.
Transmission temp sensor can cause a slip or high rpm before shifting to next gear.
your radiator has internally failed. it needs to be changed immediatly before irreversible damage is done to the transmission. flush your transmission and coolant several times.
Low transmission fluid, transmission on it's way out.
sounds like the transmission is slipping
If it is automatic and it grinds, that means you must come to a complete stop before shifting into reverse, or your transmission is no good
No but, be sure it has come to a complete stop before you put it in park.