Are you sure it'* going into OD, and it'* not just the TCC lockup that you're feeling as that fourth shift?
As a test, if you're driving down the road at around 55-65 in "3", and shift it into "OD", you should feel it shift, or hear the RPMs come down. If not, something is up.
It almost sounds like a TCC lockup shudder issue, though I didn't think that was very common at all on these older non-electronic transmissions. The lockup was very abrupt on these things. On newer models, they made it a smooth engagement, and as a byproduct, introduced more slippage... and wear.
The shudder at low speeds: Do you mean at about 35 mph, and again at about 45-50? Those would be speeds where the transmission has just shifted into a higher gear, and so the engine isn't revving very high at all. If you are in need of a tuneup, you will feel some shudder or surging at these speeds. If that'* all it is, new plugs and wires (if necessary) might solve that issue. Might solve the high-speed problem as well, if the transmission is working properly.
I'm Jon. I'm completely stock, no mods.
1993 Bonneville SSE 191,000 miles "The Work Beater" Imperfect but quite tolerable. Stock.
1996 Buick LeSabre Limited, 181,000 miles "The family car" Custom CAI, F41 Sways.
Sold/dead, but not forgotten: 1989 Bonneville LE "Bonnie, the Sport Utility Car" STB from '92-'99 Bonnie, Delco (Type II) Ignition system
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