1994 Suburban tranny acting wierd

[Kord02]

Hey ya'll, so I just picked up a 1994 chevy suburban with the 350 engine and 4l60e. When I put it in drive it feels like its stuck in second or third gear. It takes off slow like its under powered then drives fine around 20-30 then gets up to 3k-35k rpms if your faster like around 50mph. I thought the tranny was messed up so I tried putting it into 1,2,3 gears manually they all worked fine. Just wondering if i can get some help on when its not shifting when I am in drive. I called a few local shops and was told it could be the TPS sensor or ignition switch is bad and not giving the tranny the correct voltage. Any ideas or where to start?
Thanks!

 

[zora]

do you have a check engine light on?
since you have an electronic transmission to really find out whats wrong with it you need a shop with a scan tool that can run diagnostic tests.

 

[Kord02]

I dont have a check engine light on. But I will take it to get the codes read when i get back into town this week.