A 350 engine was not available in the 2001 model year; Tahoes only came with either a 4.8 liter or a 5.3 liter engine. Which engine you have will likely make a difference in the estimate; can you verify which engine you have?
To replace the rear main seal will require that the transmission first be removed for access to the rear of the engine; on this engine teh rear seal is a one piece design that is installed from the back. There is also a gasket that seals teh rear seal housing to the engine; this is usually resealed at the same time as it too can leak oil in the same area.
here is what is required:
(1) 89060436 Rear oil seal $ 20.90
(1) 12574293 Rear Oil Seal Gasket 23.35
Engine oil and filter ; estimated $25.00
The labor time required is approximately 9.9 hours (it varies a little according to exactly waht model of transmission is in your Tahoe). using a median national labor rate of $75/hr this would give an approximate labor cost of around $742.50. Labor rates do vary somewhat by area and from one shop to another, so you may want to inquire as to the shop's hourly labor rate that you intend to take the vehicle to for repair.
So, we have parts costing $69.25 and labor costing $742.50 for a total of $811.75 plus tax for this repair.