Tranny in my 'burb going bad?

[chevy_fatman]

I recently bought a 1990 4x4 1/2 ton suburban (old body style) it has a 350 and a 4 speed automatic transmission, first time I drove it back to my place I was driving around 55 MPH and all of a sudden I feel and hear this thumping sound that I could not locate where it was coming from, it also felt like it was "jumping" a bit, it did this only while I was driving on 4th. gear, so I slowed down and down shifted to 3rd gear and drove home like that no more sound or funny noises.

The tranny had been replaced by PO a couple of years before I got the 'burb and I believe it was a used one (probably got it from a junkyard) and the 'burb sat since then because his wife made him buy her a brand new suburban.

Are the internals going bad on my tranny? or does anyone what might be wrong with my 'burb?

I was going to take the 'burb to a garage and service the tranny (flush all oil and replace the tranny filter) but if the tranny is going bad this would be useless.


I would appreciate any advise you guys can give me.

Thak you. Fatman.