need help diagnosing my 02 blazer zr2

[zcarello]

i have an 02 blazer zr2 with about 113,700 miles on the odometer. the other day i was on the highway going about 60 mph when the tachometer began to tach out at about 4000rpm's. i have never had any issues with this truck, it had a little pep in its step. when i got home i checked my oil, transmission, and coolant. to my surprise the transmission dipstick actually looked a little overfilled, the oil was bone dry, and the coolant was low. i added coolant and oil. now while i am driving there is little to no power in the truck. on main roads its sufficient but seems to be slow to shift, and on the highway forget about accelerating to change lanes, you can hammer the gas and the truck tachs out without downshifting. the only way i can increase my speed from 45 to anywhere near 60 is manually downshift and lift off the throttle when i want it to shift then ease back into the throttle, eventually it will shift into gear. what could be wrong? i amfiguring its the transmission because it seems pretty obviouse but is it a something small wrong or do i have replace the whole tranny??? or is the computer just not recignizing that it should shift?