I have the same truck, except 4:11's, mine was an absolute slug!!!!!!!!
UNTIL... .
I got guages, 3k GSK, #10 plate slid almost full forward, and now my trans is done and loving life!!
I can tell you right now even without seeing the hills, i can pull the same load, same everything, up that hill 65 to 70 MPH and gain speed without watching my EGT guage.
My truck is setup about perfect for towing anything and everything there is to be towed!
With that in mind i would highly suggest spending the cash on the above items as well as Valve Adjustment, Fuel hose replacement (in and out), BHAF, new fuel filter, and either stretch or replace fuel pressure spring. O and after all of the above, you might want to start checking into a trans or clutch.
Good Luck
Curtis