'96 F250 PSD Can't climb a curb

[17YRSOFCOLLEGE]

Hey folks the title pretty much says it all but I'll go into more details here.
I bought the truck about 5000 miles ago with 335,000 miles on it. It is a crew cab, 4x4 with 6" rough country lift and 12.5/35/17 tires.
Truck has barely 5k on the lift.
No modifications to motor that I know of whatsoever, was told that the transmission was rebuilt at 300,000 miles. The transmission shifts fine but the fluid looks dark and should probably be changed soon.
The truck has tons of power once its moving past 5 mph our so but from a stop its VERY slow. I thought that it was odd so I pulled it up against a curb ( about 8" tall) and it would not climb over the curb!
The engine RPMs would hold at ~1800 RPMs and never go over, preventing the truck from achieving high enough revs to climb. There is no transmission slipping that I can feel at all.
This has me and my buds stumped...I would think fuel delivery but it rips (relatively for a big truck) on tbd highway and a soon as it gets over the previously mentioned 5mph so I'm ruling that out.
What do you think?
Since I've bought the truck I've done 2 oil changes, the fuel pump ( was leaking ), front axle U joints, headlights, alternator, serpentine belt, tensioner pully, both batteries, oh God so much other cleaning crap.
Truck is one of the cleanest 335k mile trucks I've seen...
I just want this truck to work properly.
Any ideas would be really appreciated...I'm into this truck for appropriately $10,000 now lol

[17YRSOFCOLLEGE]

[17YRSOFCOLLEGE]

[FurbyPower]

You really dont have any issues with the truck. If its 4wd put it in 4lo and it will crawl no problem. With the lift and tires your factor gear ratio is way too tall. If you want it to perform like it did before the lift you are going to have to regear the truck.

[17YRSOFCOLLEGE]

Thank you very much. I sincerely appreciate your response. That's very nice of you and I just tried the 4 low and it worked flawlessly

[strokin95style]

It may have 3:55's, with that tire size, I would run either a 4:10

Sent from Nater's Android phone.

[strokin95style]

It may have 3:55's, with that tire size, I would run either a 4:10 or a 4:56, depending on how far you drive and how much power you want.

Sent from Nater's stupid Android phone.