Sprinter Gearbox Slippage Diagnosis

Gear wear in the transfer case is plausible given the symptoms. Have you inspected the transfer case oil recently? Atypical transfer case oil coloration or the presence of metallic particles would strongly suggest internal damage. Consider a thorough inspection for external leaks as well.

I haven't checked the transfer case oil itself. Where should I be looking for potential oil leakage points on the transfer case?

Leakage often manifests around the input and output shaft seals, as well as the case halves. Given the symptoms and mileage, internal gear wear leading to slippage is highly probable. A workshop inspection is advisable to confirm the diagnosis and estimate repair costs.

I appreciate the insight. The workshop confirmed defective gears in the transfer case. Repair cost came to 330€. Thanks for pointing me in the right direction.