Anyone know how I can fix my Stampede?

So I've had an original 2wd Stampede with a brushless motor for like 6 or 7 years and I've never had any major issues that couldn't be simply resolved. However now I need some help. I noticed one day while running it normally that the drivetrain was making a clicking noise when on the throttle. I immediately picked up the truck to get a closer look and noticed that when there's no force on (wheels in the air) the sound goes away and all seems fine. It got worse and worse until now there's a considerable loss in off the line acceleration and top end speed.

Of course I took apart everything in the drivetrain to try to find the source of the problem. Pinion looks okay, replaced the spur gear, replaced the slipper clutch and pads, even replaced the differential. And still the clicking and lack of power is as evident as ever!

I'm running out of ideas, anyone know where I should look next? Have you guys ever experienced anything similar?