what's that burning smell?

[99' Powerstroke]

I was towing a skid steer....total weight with trailer was pushing 12,000 lb up a hill that has about a 1 mile grade. I have a six speed manual and was lugging it up the hill in fourth gear. I was doing about 45-50 mph up the hill and when I got to the top I could smell something burning. I know I shouldn't lug it but I didn't really feel like downshifting. I don't have a pyro guage but I don't have any chip either. I only have a 4 inch turbo back and afe intake. I also have a 1.0 exhaust housing. Was I pushing high EGTs and melting something? I should get a pyro guage.

[329373]

How many miles are on your clutch?
JB Bergman

[99' Powerstroke]

143000 miles on the truck, and I'm pretty sure it's the original clutch. I've actually been thinking of replacing it because it's probably due for one. I can't feel the clutch slipping when I'm accelerating from low gear and if it's going to burn that's when I would expect it.