Your transmission is electronically controlled by the ECM, what determines the shift points and firmness are a few sensors that can be check easily. Here are the common things you need to know and addressed to what your have described.
Common failures on 4L60E, symptom; cause; any possible repair:
1. 1-2 shift does not happen at WOT until you let off the gas: Best case: try replacing the TPS. Middle case: leak in the 2nd gear apply circuit (servo assembly or 1-2 accumulator). Doublecheck by using the pressure gauge and watch for a big drop when the PCM commands 2nd gear. Worst case: poor line pressure rise.
2. 1-2 Shift shudder at WOT; delayed or abnormal 1-2 shift; There's a problem ONLY on the 1-2 shift: 1-2 accumulator piston cracked or stuck cocked in the bore. Check the yellow spring inside the accumulator housing for breakage. Also, if the accumulator housing walls are scored, the housing must be replaced. Easy fix in the pan.
3. 1-2 shift is delayed and harsh, may not shift into OD: Check TPS for smooth and linear electrical response over the entire range of motion. If not, replace.
4. Shift suddenly become very hard. When going from park to either reverse or drive it slams into gear. 1-2 and 2-3 shifts are also harsh. All gears seems to work. Problem may be intermittent: Check TPS for smooth electrical response. If the response is jumpy or erratic at all, replace TPS.
Best thing to do now since you have been driving the transmission in that condition is to have it service. Replace the transmission fluid and filter and also it's a good thing to replace the boost valve every 100,000 miles to ensure you getting the proper transmission fluid pressure. Low transmission fluid and pressure will shorten the transmission's life, you can replace the boost valve with the same boost valve from a corvette and they go for around $45.00 and if you would like a firmer shift you can replace the servo's and they go for around $45.00 also. I replaced the servo's and boost valve in my 1996 jimmy at the first transmission service at 45,000 miles and replace the boost valve every 100,000 miles intervals. She is now at 386,000 miles and still has a firm shift with no problems. It's common for the springs in the boost valve to weaken after so many years causing the transmission line pressure to drop causing the transmission to be lazy and seam to slip.
Good luck and start with checking the throttle position sensor, and replace if worn out. Also if your going to have the transmission serviced, have them check the transmission line pressure. Base pressure at idle in P,N,OD should be around 70psi and around 140psi in R. Max line pressure should be at least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump is worn out. If max pressure is low, check EPC solenoid. If reverse is slow to engage, and line pressure does not come up to at least 130psi in reverse, replace the boost valve with a Transgo 0.500" boost valve or one from a corvette if you like to keep it OEM (easy fix in pan).
