4R100 Transmission in Early '99 F250 SD PSD.

First OBDII scan I ran showed DTCs 0733 and 0755 for gear ratio/transmission solenoids.
Cleared all DTCs. 0755 (SSB) persists, 0733 (incorrect gear ratio) has not set again.
Starting in Drive requires feathering the accelerator to force an upshift, as one might expect with 0755 (SSB fault). It goes straight to 3rd.
Manually shifting from 1->2->D results in smooth, firm shifts through ALL gears. Downshifts manually, or staying in D, seem to occur without issue and as expected for an otherwise seemingly healthy transmission. OD works as expected.
I do not have a breakout box to do electrical tests per svc man.
Careful visual inspection of the wiring harness (covered with cable protector/loom) does not reveal any concerns. The harness runs along the frame as built, is not pinched nor chafed.
Is there an alternate methodology for testing the circuit without the breakout box?

Given the specifics above, is the most likely/sole culprit the solenoid pack inside the transmission?
(I feel confident it is and is the only cause at this point but, I value the insight of many of the members here at FTE.)
Are there any other items that may be contributory to the shift issue that I've possibly overlooked?
(Mark, if you happen by this post, I know visual inspection alone is not sufficient to find a bad wire. )

I haven't dropped the pan for inspection or done a fluid change yet since I've been trying to nail down this specific issue before I start spending money on it.

 

A breakout box makes it easier, but you don't have to have one. On the other hand, neither code indicates an electrical problem, so I don't expect you will find an electrical problem. The problem is a bad shift solenoid.

 

First, thank you for taking the time to read and reply.
Part ordered. Expected delivery 9/22-9/25.
I'll post back here with the results when I complete the job for anyone interested in following up.
I feel I'd be remiss if I didn't also drop the main valve body for inspection while I'm in there. Certainly couldn't hurt as a PMC (Preventative Maintenance Check).