I have an 04 Silverado with a 4l60e. A few months back I noticed my truck has had a ever worsening 1-2 shift, slow to engage and a sudden very firm shift. Forums have dubbed it the dreaded slide bump. So to correct the problem I bought and installed a Transco 4l60e HD2 shift kit. Upon careful installation and a test drive I notice that I had no 4th gear, all shifts were fine but the one into 4th, it would run at +-2600rpm and just stay there. Having had minor trouble with the 4th piston on the servo when I installed it, I matched cereal numbers and got another servo all together, this servo had a much easier function, but did not solve my problem. and its throwing p0751 code. So I got to thinking, after watching many videos and reading experiences on forums I noticed in some 2nd pistons the old spring was replaced with a new spring, while in mine the instructions called for both springs to be placed inside. So my question to you: what could be causing my problem? Could it be that I shouldn't have put both springs in?

 

This does sound like a servo and/or band problem.
The first step is to measure the servo end play.
Perhaps using a crow bar, hold a small ruler next to the servo and press on the cover with the crow bar until it bottoms out. The correct end play is 1/16"-1/8". Around 3/16" the band may begin to slip under WOT and by 1/4" you will lose 2nd and/or 4th gear. It seems with with stock servos most people lose 2nd before they lose 4th, but with aftermarket/billet servos I have heard it is the other way around.

For a quick guess, I will say that that band is simply worn.

 

You didn’t mention that you had problems with 4th gear before the kit install so I assume it was shifting fine before? On page 1 step one, did you install one or both Gold band adjust shims and did you install the 4th piston with the tall **** towards the servo cover?