Sounds like what you're saying is the trans won't shift into OD without setting codes.

Could this be the infamous stripped splines on the 4th gear clutch hub?

I'd say drop the tranny pan and look for metal shavings. Also, pull the ATF filter, cut it open and look for the same.

Hmm, then I have 2 problems, no 4th gear (overdrive) and no lockup. Bummer. :?

Wait, I may have misunderstood your original post. Are you going into 4th gear (Overdrive)?

When shifting from 3rd to "D'" the engine rpms remain at approx. 2500 @ 65 mph. What should the normal rpm'* be in "D"?

I have an 87 Fiero w/ a 3800 Series 1 mated to a 4t60. It cruises 65 at about 1700 rpm.

You have to use your axle ratio and work backwards...internally, 4th gear is 0.705:1, 3rd is 1.000:1. But you also have a chain drive and differential. Look up the your transmission'* RPO code and that will tell you what the chain gears and diff ratios should be on your transaxle. Using that, you can compute the engine rpm in 4th with the TCC locked up (engine rpm=input shaft rpm..almost, the TCC is pulsed on this transmission, allowing a controlled amout of slip...not much, but there none the less)

If you are used to the feel of the of the 4T60 from your Fiero, you may be a little missled by the feel of TCC lockup on a 4T60E..there'* no soft "thunk" when the TCC engages. The pulse modulation of the TCC solenoid softens that out...we can't have any strange thunks and bangs to disturb the passengers

From this, I would say you are not going into OD. At 65 mph, you should be around 1700 (give or take 50) rpm in OD with TCC lockup.

Does the 2000 Bonnie by chance have some kind of "overdrive off" button option?

No, not by chance nor by design.

That'* on the Ford F250 gear selector lever.

The only way for the transmission to go into 3rd with the selector in "D" is when going "35 mph or more, push the accelerator all the way down." (per Owner'* Manual) or at "40-55 mph quickly increase the throttle angle to greater than 50%" (per Service Manual) .

What about this?

"Occasional problems with this vehicle are failures of the Transmission Pressure Control Solenoid (PCS) and the connections at the Transmission Pass-Thru Connector. Problems with the Transmission PCS are identified in TSB # 00-07-30-002B. Problems with the connections of the Transmission Pass-Thru Connector are identified in TSB #02-07-30-022B. The cost to replace the Transmission PCS is estimated at $45.61 for parts and $273.00 for labor. The cost to repair the Transmission Pass-Thru Connector is estimated at $10.00 for parts and $65.00 for labor. All prices are estimates based on $65.00 per flat rate hour and do not include diagnostic time or any applicable sales tax."

(from msn.autos 2000 Pontiac Bonneville Reliability)

OD should be 1900 at about 70mph.

My money is a stripped 4th gear clutch shaft.

I think you may be correct. Meanwhile, a new Scangauge II is on it'* way here, thanks to E-bay.