Transmission Issue

[rambling will]

2001 georgetown 32.5 V-10 - Just this afternoon on our way thru the White Mountains as the transmission shifted down from the normal drive gear to assist in a climb we began going slower and slower rather than moving along up the incline. The engine races normally as if it has done what its supposed to do but there is no increase in forward moving power. Also there is a pinging/high pitch rattling noise. seems fine in the regular drive gear no noticible odd difference from normal operation jsut when shifting down on the run. Stopped put the tranny in all gears and stepped on the gas holding the brake down and there does not seem to be slippage. Transmission fluid level is full. Any ideas or has any one experienced this issue? COuld this just be bad gas or is it a tranny issue. Everything seems fine and normal in regular drive gear. Thanks

[cfsoistman]

Were you towing anything? Does the 2001 have a Tow/Haul button to over ride overdrive? Air Filter checked lately and does the 2001 require a higher octane fuel? You may have gotten a bad tank of fuel. This may have caused the pinging. Have you tried any octane booster cleaner? How many miles on the unit? Do you keep your tank full? I've heard of condesation forming in tanks less than half full and then it's in the fuel or it may be time for a tune up.
My 2007 wasn't too good on steep hills unless I really pushed it hard and the transmission and engine worked really hard to get up the hill. I installed a Banks Cold Air Intake System with a big time K&N air filter and much larger intake box openings. It really made a difference. I can buzz up the hills even when towing without putting much of a strain on the motor and tranny..