Slow acceleration on my used 2008 Silverado. Having to push hard on the accelerator before I can get to cruising speed. Not sure if this is due to the limits of the 4 speed automatic or what. Please help

Connecting a pro grade scan tool computer will tell you the DTC Code and that tells you what the problem is and how to fix it. The 'Wrench Light', properly the 'Engine Malfunction Light' is there to tell you that you have a problem and the scan tool tells you what it is?

Cruise control only maintains speed by utilizing the accelerator. To speed up it increases it and during slowing is removes it. The cruise control will not apply brakes in the event of a steep downhill run. In this case the vehicle will start to accelerate in the same manner as if the cruise control was not engaged. But in theses cases, most cruise controls, once the speed exceeds a certain amount above the set speed, will auto-disengage as a safety feature.

Cruise Control
Any given speed above approximately 22 mph (35 km/h) can be maintained with the cruise control by operating the lever.
-see photo

1. Accelerate and set: Lift lever briefly to set speed. Hold lever up to accelerate.
2. Decelerate and set:
   Depress lever briefly to set speed. Hold lever down to decelerate.
   Normally the vehicle is accelerated to the desired speed with the accelerator. Speed is set by briefly pushing
   the lever to position 1 or 2. The accelerator can be released. The speed can be increased (e.g. for passing)
   by using the accelerator. As soon as the accelerator is released, the previously set speed will be resumed automatically. If a set speed is to be increased or decreased slightly, e.g. to adapt to the traffic flow, hold lever in position 1 or 2 until the desired speed is reached, or briefly tip the control lever in the appropriate direction for increases or decreases in 0.6 mph (1 km/h) increments. When the lever is released, the newly set speed remains.
3. Canceling

To cancel the cruise control, briefly push lever to position 3.
When you step on the brake or the vehicle speed drops below approx. 22 rnph (35 km/h), for example when driving upgrade, the cruise control will be canceled.
4. Resume
If the lever is briefly pushed to position 4 when driving at a speed exceeding approx. 22 mph (35 km/h), the vehicle resumes the speed which was set prior to the cancellation of the cruise control. The last memorized speed is canceled when the key in the steering lock is turned to position 1 or 0.
Note:
If the engine does not brake the vehicle sufficiently while driving on a downgrade, the speed you set on the cruise control may be exceeded and you may have to step on the brake pedal to slow down.
As soon as the grade eases, the cruise controlled speed will again be maintained as long as the brakes were not previously applied, or the lever may be used to resume the previously set speed if the brakes were applied.


http://www.benzworld.org/forums/attachments/w202-c36-c43/153145d1197313761-pdf-owners-manual-w202-1995-2000-1999-c280_owners_manual.pdf

ANY SERVICE LAMPS ON?

clutch plate and pressure plate could be worn out.

Once you have turned your cruise control on accelerate to desired speed press SET button on right hand side of steering wheel before lifting foot from accelerator, then remove foot from accelerator. pushing RESUME button will deactivate cruise control without turning it off. Then when clear of traffic push resume again to activate cruise control. It will automatically resume to previous set speed.

cruise controls are funny things
you have to accelerate to a speed then turn it on and press set
It will cruise at that speed
if you touch the brake it will unset and you will have to press on the accelerator again to the speed and pres resume
if you want to accelerate to pass just press the accelerator until you have passed , then just take your foot off the accelerator and it will coast until the set speed
if you change gears , it will unset
if you touch the brake , it will unset
if you have it set and find the speed to fast , hold the set button and it will slow down to the speed you want
let the button go and it will set at the speed
if you want to go faster than the set speed press the resume button and it will speed up until you get the speed you want
from your description of the problem , it appears that you have misread the instructions in the owners manual

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Generally... There is ON/OFF - SET - ACCELERATE/RESUME, COAST/DECELERATE. Cruise is available at speeds above 35mph or so. Sometimes there is no main on/off. Cruise on a turn signal stem has a button that pushes in on the end, push when at desired speed. resume/accelerate slides to right, is spring loaded and return left when released. the same lever when slid left is coast/decelerate. Momentarily sliding accelerate and releasing increases crusing speed by a couple mph. If brakes are used cruise disengages, sliding resume returns you to cruise speed after braking. Coast/decelerate disengages cruise, when at desired lower speed hit set. On some, hitting set not needed, when coast is released speed is maintained. Variations may be up/ down instead or may be steering wheel face mounted. Please rate answer. Duct tape is like the force, it has a dark side.. a light side.. and holds the universe together!

Turn key, on, off, on, off, on & leave it on (not start) & count check engine flashes. 2 digit codes, for example: flash, flash, flash, pause, flash,flash., is code 32