86,000 miles on original fluid. same with the differential. is the differential as tricky to change as the tranny?

First welcome to the forum! No one has chimed in...

The CLK chassis is near identical to the W210 E-Class. Except for the location of the tranmission computer(The 722.6 is a electronic transmission). The CLK has it behind the front vertical passenger carpet behind a steel plate. The W210 has the transmission computer in the computer coffin. It is important to know this as ALL 722.6 equipped cars leak past the circular connector seals, and the transmission fluid will wick-up into the transmission computer and cause all kinds of problems. In most cases it put the car in limp mode(2nd gear and reverse). The computer can be cleaned with electrical spray contact cleaner, but watch out for the shops that want to charge for the harness and computer. Those charges will put your bill around $2k. If you need the process to clean the computer, let me know?

I like to change the transmission fluid every 30k miles if the pan fluid is only replaced, and 40k miles with pan and torque converter.

As you realized their is no dip stick, I really suggest you buy the dipstick from Pelican parts. Do not do the measure what I took out, and I will put it back in routine.. Not safe to do that.

1999 Mercedes-Benz CLK430 Base Coupe - Automatic Transmission - Page 2

Pelican parts has a great DIY on the fluid change/filter change/circular connector change.

Pelican Technical Article: Mercedes Benz - Automatic Transmission Fluid Change

Here is a excellent thread on the circular connector:

My attempt at replacing the transmission plug. - Mercedes-Benz Forum

Everything you ever wanted to know on the 722.6:

Power Train, Drive Line (Engine, Transmission, Differential, etc) - Mercedes-Benz Forum

On your Differential same as the W210. Always loosen the filler cap first. Some say to replace the threaded caps. Another great thread from pelican:

Pelican Technical Article: Mercedes Benz - Differential Oil Change

The transmission fluid is very particular fluid. Make sure it is for 722.6 or NAG1(the Chrysler name for the 722.6). Do not use fluid that says "import" or "European Import"..

Get that dipstick it offers a peace-of-mind. Keep checking your fluid for a few days. Remember, Transmission fluid is checked with the engine running. Insert the dipstick until it stops. Donot try to ram the the whole springy coil in or you will break or bend the plastic tip of the testing dipstick..

All the best,

Martin