stumped on this one 92 gmc

hi new problem 1992 gmc 5.7 4x4 167,000 miles on it about 40 miles hour my transmision starts to go between 3 and 4 gear have done a lot work chasing this problem i have put new rear main seal egr valve mass air flow sensor throtel pistion sensor fuel relay new altator checked to see if any wires are bad im stuped with this one if any you old timers can help please oh my truck did throw a code 54 ob1 book says it is fuel relay changed thanks for your time :banghead::runaround:

Do you mean it shifts into fourth at 40mph?
I believe that's normal, at least it is for my '96 GMC.
I usually leave it in 3rd until I get above 50mph then I will shift it into fourth.
If I am towing I will leave it in third unless I'm on level ground or traveling above 65mph.

thanks for the reply as you are driving sit there and go back and fourth