stock 700r4 issue

[1ndatrunk]

Ok, so here I am again with a problem. You guys have been really good sports a about answering my noobish questions, but I have more. This time in reference to some highway issues I'm having with my stock 88 Suburban.

TBI, 148k miles, unknown rear gearing, 350v8. I can't seem to get this thing to cruise down the highway in od at any speed higher than 55. Past 55 it just sounds like my engine is screaming down the highway and never goes into od. If I'm going slower than 60, it does ok, but with the SLIGHTEST bit of pressure it will drop down to 3rd gear. My gas pedal is also very tight at all positions. I was told it could be my tv cable being too tight or a dirty filter, but I wanted to ask you guys first before I throw money at the problem. I swapped my rear a month ago with an 83 suburban from the junk yard, but the RPO sticker was missing from the glove box, so gears are unknown

Could all of these issues be the cause of one thing, or are these all different problems? Thanks in advance for any help you can give.