Check your tires are all the same sizes and are all properly inflated. The triangle light doesn't mean there is a fault, it means the system is actively seeing wheel slip and locking the diff, or attempting to lock in the case of a worn diff. This occurring only at higher speeds is b/c at lower speeds it doesn't interpret wheel diameter discrepancies as slip. At high speed there is enough difference that it thinks its slipping and is keeping the system engaged.

If you can confirm that all the tires are the same measured diameter only then would I look at the wheel speed sensors.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff